Qué hay de nuevo en Python 2.6

Autor

A.M. Kuchling (amk arroba amk.ca)

Este articulo explica las nuevas características en Python 2.6, publicado el 1 de Octubre de 2008. El calendario de publicación se describe en PEP 361.

El tema principal de Python 2.6 es preparar el camino de migración a Python 3.0, un importante rediseño del lenguaje. Siempre que sea posible, Python 2.6 incorpora nuevas características y sintaxis de 3.0 mientras sigue siendo compatible con el código existente al no eliminar características o sintaxis más antiguas. Cuando no es posible hacer eso, Python 2.6 intenta hacer lo que puede, agregando funciones de compatibilidad en el módulo future_builtins y un interruptor -3 para advertir sobre usos que dejarán de ser compatibles en 3.0 .

Se han agregado algunos paquetes nuevos importantes a la biblioteca estándar, como los módulos multiprocessing y json, pero no hay muchas características nuevas que no estén relacionadas con Python 3.0 de alguna manera.

Python 2.6 también incluye una serie de mejoras y correcciones de errores en el código fuente. Una búsqueda en los registros de cambios encuentra que se aplicaron 259 parches y se corrigieron 612 errores entre Python 2.5 y 2.6. Es probable que ambas cifras estén subestimadas.

Este artículo no intenta proporcionar una especificación completa de las nuevas características, sino que proporciona una conveniente descripción general. Para obtener detalles completos, debe consultar la documentación de Python 2.6. Si desea comprender la justificación del diseño y la implementación, consulte el PEP de una característica nueva en particular. Siempre que sea posible, «Qué hay de nuevo en Python» enlaza con el elemento de error / parche para cada cambio.

Python 3.0

El ciclo de desarrollo de las versiones 2.6 y 3.0 de Python se sincronizó, y las versiones alfa y beta de ambos lanzamientos se realizaron los mismos días. El desarrollo de 3.0 ha influido en muchas características de 2.6.

Python 3.0 es un rediseño de Python de gran alcance que rompe la compatibilidad con la serie 2.x. Esto significa que el código Python existente necesitará alguna conversión para poder ejecutarse en Python 3.0. Sin embargo, no todos los cambios en 3.0 rompen necesariamente la compatibilidad. En los casos en que las nuevas funciones no provoquen la rotura del código existente, se han actualizado a 2.6 y se describen en este documento en el lugar correspondiente. Algunas de las características derivadas de 3.0 son:

• El método __complex__() para convertir objetos en un número complejo.

• Sintaxis alternativa para detectar excepciones: except TypeError as exc.

• La adición de functools.reduce() como sinónimo de la función incorporada reduce().

Python 3.0 agrega varias funciones integradas nuevas y cambia la semántica de algunas integradas existentes. Las funciones que son nuevas en 3.0 como bin() simplemente se han agregado a Python 2.6, pero las funciones existentes no se han cambiado; en cambio, el módulo future_builtins tiene versiones con la nueva semántica 3.0. El código escrito puede ser compatible con 3.0 haciendo from future_builtins import hex, map según sea necesario.

Un nuevo modificador de línea de comandos, -3, habilita advertencias sobre características que se eliminarán en Python 3.0. Puede ejecutar código con este modificador para ver cuánto trabajo será necesario para migrar el código a 3.0. El valor de este modificador está disponible para el código Python como la variable booleana sys.py3kwarning, y para el código de extensión C como Py_Py3kWarningFlag.

Ver también

Las series 3xxx de PEP, que contienen propuestas para Python 3.0. PEP 3000 describe el proceso de desarrollo de Python 3.0. Empiece con PEP 3100 que describe los objetivos generales de Python 3.0, y luego explore los PEPS con números más altos que proponen características específicas.

Cambios en el proceso de desarrollo

Mientras se desarrollaba 2.6, el proceso de desarrollo de Python experimentó dos cambios significativos: cambiamos del seguidor de incidentes (issue tracker) de SourceForge a una instalación personalizada de Roundup, y la documentación se convirtió de LaTeX a reStructuredText.

Nuevo seguidor de incidentes: Roundup

Durante mucho tiempo, los desarrolladores de Python estaban cada vez más molestos por el seguidor de errores de SourceForge. La solución alojada en SourceForge no permite mucha personalización; por ejemplo, no fue posible personalizar el ciclo de vida de los problemas.

Por lo tanto, el comité de infraestructura de la Python Software Foundation publicó una convocatoria sobre el seguimiento de incidentes, pidiendo a los voluntarios que configuren diferentes productos e importen algunos de los errores y parches de SourceForge. Se examinaron cuatro seguidores diferentes: Jira <https://www.atlassian.com/software/jira/> __, Launchpad <https://launchpad.net/> __, Roundup y Trac. El comité finalmente se decidió por Jira y Roundup como los dos candidatos. Jira es un producto comercial que ofrece instancias alojadas sin costo para proyectos de software libre; Roundup es un proyecto de código abierto que requiere voluntarios para administrarlo y un servidor para alojarlo.

Después de publicar una llamada para voluntarios, se configuró una nueva instalación de Roundup en https://bugs.python.org. Una instalación de Roundup puede alojar varios seguidores, y este servidor ahora también aloja seguidores de problemas para Jython y para el sitio web de Python. Seguramente encontrará otros usos en el futuro. Siempre que sea posible, esta edición de «Qué hay de nuevo en Python» se vincula al elemento de error/parche para cada cambio.

Upfront Systems <http://www.upfrontsoftware.co.za> __ de Stellenbosch, Sudáfrica, proporciona amablemente el alojamiento del seguidor de errores de Python. Martin von Löwis se esforzó mucho en importar errores y parches existentes desde SourceForge. Sus scripts para esta operación de importación se encuentran en http://svn.python.org/view/tracker/importer/ y pueden ser útiles para otros proyectos que deseen pasar de SourceForge a Roundup.

Ver también

https://bugs.python.org

El seguidor de errores de Python.

http://bugs.jython.org:

El seguidor de errores de Jython.

http://roundup.sourceforge.net/

Descargas y documentación de Roundup.

http://svn.python.org/view/tracker/importer/

Scripts de conversión de Martin von Löwis.

Nuevo formato de documentación: texto reestructurado con Sphinx

La documentación de Python se escribió usando LaTeX desde que el proyecto comenzó alrededor de 1989. En la década de 1980 y principios de la de 1990, la mayor parte de la documentación se imprimió para su estudio posterior, no se vio en línea. LaTeX fue ampliamente utilizado porque proporcionaba una salida impresa atractiva sin dejar de ser sencillo de escribir una vez que se aprendían las reglas básicas de marcado.

Hoy en día, LaTeX todavía se usa para escribir publicaciones destinadas a la impresión, pero el panorama de las herramientas de programación ha cambiado. Ya no imprimimos montones de documentación; en su lugar, lo navegamos en línea y HTML se ha convertido en el formato más importante para dar soporte. Desafortunadamente, convertir LaTeX a HTML es bastante complicado y Fred L. Drake Jr., el editor de documentación de Python desde hace mucho tiempo, pasó mucho tiempo manteniendo el proceso de conversión. De vez en cuando, la gente sugeriría convertir la documentación a SGML y luego a XML, pero realizar una buena conversión es una tarea importante y nadie comprometió el tiempo necesario para terminar el trabajo.

Durante el ciclo de desarrollo 2.6, Georg Brandl se esforzó mucho en crear una nueva cadena de herramientas para procesar la documentación. El paquete resultante se llama Sphinx y está disponible en http://sphinx-doc.org/.

Sphinx se concentra en la salida HTML, produciendo HTML moderno y con un estilo atractivo; la salida impresa todavía se admite mediante la conversión a LaTeX. El formato de entrada es reStructuredText, una sintaxis de marcado que admite extensiones y directivas personalizadas que se usa comúnmente en la comunidad de Python.

Sphinx es un paquete independiente que se puede usar para escribir, y casi dos docenas de otros proyectos (enumerados en el sitio web de Sphinx) han adoptado Sphinx como su herramienta de documentación.

Ver también

Documentando Python

Describe cómo escribir para la documentación de Python.

Sphinx

Documentación y código para la cadena de herramientas Sphinx.

Docutils

El analizador sintáctico y el conjunto de herramientas reStructuredText subyacentes.

PEP 343: La sentencia “with”

La versión anterior, Python 2.5, agregó la instrucción “with” como una característica opcional, para ser habilitada por una directiva from __future__ import with_statement. En 2.6, la instrucción ya no necesita estar habilitada especialmente; esto significa que ahora with es siempre una palabra clave. El resto de esta sección es una copia de la sección correspondiente del documento «Qué hay de nuevo en Python 2.5»; si está familiarizado con la declaración “with” de Python 2.5, puede omitir esta sección.

La sentencia “with” resuelve el código que anteriormente usaría bloques try…finally` para garantizar que el código de limpieza se ejecute. En esta sección, discutiré como se usará comúnmente la declaración. En la siguiente sección, examinaré los detalles de la implementación y mostraré cómo escribir objetos para usar con esta declaración.

La sentencia “with” es una estructura de control de flujo cuya estructura básica es:

with expression [as variable]:
    with-block

La expresión se evalúa y debe dar como resultado un objeto que admita el protocolo de administración de contexto (es decir, tiene los métodos __enter__() y __exit__()).

El objeto __enter__() se llama antes de que se ejecute with-block y, por lo tanto, se puede ejecutar código de configuración. También, si se proporciona, puede retornar un valor que esté vinculado al nombre variable. (Tenga en cuenta que a la variable no se le asigna el resultado de la expression).

Una vez finalizada la ejecución de with-block, se llama al método __exit__() del objeto, incluso si el bloque generó una excepción y, por lo tanto, puede ejecutar código de limpieza.

Algunos objetos estándar de Python ahora admiten el protocolo de administración de contexto y se pueden usar con la sentencia “with”. Los objetos de archivo son un ejemplo:

with open('/etc/passwd', 'r') as f:
    for line in f:
        print line
        ... more processing code ...

Después de que se haya ejecutado esta sentencia, el objeto de archivo en f se habrá cerrado automáticamente, incluso si el bucle for generó una excepción en la mitad del bloque.

Nota

En este caso, f es el mismo objeto creado por open(), porque file.__enter__() retorna self.

Los locks y las condiciones variables del módulo threading también admiten la sentencia “with”:

lock = threading.Lock()
with lock:
    # Critical section of code
    ...

El lock se adquiere antes de que se ejecute el bloque y siempre se libera una vez que este se completa.

La función localcontext() en el módulo decimal facilita guardar y restaurar el contexto decimal actual, que encapsula la precisión deseada y las características de redondeo para los cálculos:

from decimal import Decimal, Context, localcontext

# Displays with default precision of 28 digits
v = Decimal('578')
print v.sqrt()

with localcontext(Context(prec=16)):
    # All code in this block uses a precision of 16 digits.
    # The original context is restored on exiting the block.
    print v.sqrt()

Escribiendo gestores de contexto

Por detrás, la sentencia “with” es bastante complicada. La mayoría de las personas solo usarán “with” en compañía de objetos existentes y no necesitan conocer estos detalles, por lo que puede omitir el resto de esta sección si lo desea. Los autores de nuevos objetos deberán comprender los detalles de la implementación subyacente y deben seguir leyendo.

Una explicación de alto nivel del protocolo de gestor de contexto es:

• La expresión se evalúa y debería dar como resultado un objeto llamado «gestor de contexto». El gestor de contexto debe tener los métodos __enter__() y __exit__().

• Se llama al método __enter__() del gestor de contexto. El valor retornado se asigna a VAR. Si no hay una cláusula as VAR, el valor simplemente se descarta.

• Se ejecuta el código en BLOCK.

• Si BLOCK lanza una excepción, se llama al método __exit__() del gestor de contexto con tres argumentos, los detalles de la excepción (type, value, traceback, los mismos valores retornados por sys.exc_info(), que también puede ser None si no se produjo ninguna excepción). El valor de retorno del método controla si se vuelve a generar una excepción: cualquier valor false vuelve a lanzar la excepción, y True resultará en inhibirla. Rara vez querrá suprimir la excepción, porque si lo hace, el autor del código que contenga la sentencia “with” nunca se dará cuenta de que algo salió mal.

• Si BLOCK no lanzó una excepción, el método __exit__() continúa llamándose, pero type, value y traceback son todos``None``.

Pensemos en un ejemplo. No presentaré un código detallado, solo bosquejaré los métodos necesarios para una base de datos que admita transacciones.

(Para las personas que no están familiarizadas con la terminología de la base de datos: un conjunto de cambios en la base de datos se agrupa en una transacción. Las transacciones pueden confirmarse, lo que significa que todos los cambios se escriben en la base de datos, o deshacerse, lo que significa que todos los cambios se descartan y la base de datos no ha cambiado. Consulte cualquier libro de texto de base de datos para obtener más información.)

Supongamos que hay un objeto que representa una conexión de base de datos. Nuestro objetivo será permitir que el usuario escriba un código como este:

db_connection = DatabaseConnection()
with db_connection as cursor:
    cursor.execute('insert into ...')
    cursor.execute('delete from ...')
    # ... more operations ...

La transacción debe confirmarse si el código del bloque se ejecuta sin problemas o revertirse si hay una excepción. Aquí está la interfaz básica para DatabaseConnection que asumiré:

class DatabaseConnection:
    # Database interface
    def cursor(self):
        "Returns a cursor object and starts a new transaction"
    def commit(self):
        "Commits current transaction"
    def rollback(self):
        "Rolls back current transaction"

El método __enter__() es bastante fácil, ya que solo tiene que iniciar una nueva transacción. Para esta aplicación, el objeto cursor resultante sería un resultado útil, por lo que el método lo retornará. Luego, el usuario puede agregar as cursor a su sentencia “with” para vincular el cursor a un nombre de variable.

class DatabaseConnection:
    ...
    def __enter__(self):
        # Code to start a new transaction
        cursor = self.cursor()
        return cursor

El método __exit__() es el más complicado porque es donde se debe realizar la mayor parte del trabajo. El método debe verificar si ocurrió una excepción. Si no hubo excepción, la transacción se confirma. La transacción se revierte si hubo una excepción.

En el siguiente código, la ejecución simplemente caerá al final de la función, retornando el valor predeterminado None. None es falso, por lo que la excepción se volverá a lanzar automáticamente. Si lo desea, puede ser más explícito y agregar una sentencia return en la ubicación marcada.

class DatabaseConnection:
    ...
    def __exit__(self, type, value, tb):
        if tb is None:
            # No exception, so commit
            self.commit()
        else:
            # Exception occurred, so rollback.
            self.rollback()
            # return False

El módulo contextlib

El módulo contextlib proporciona algunas funciones y un decorador que son útiles al escribir objetos para usar con la sentencia “with”.

El decorador se llama contextmanager(), y te permite escribir una única función generadora en lugar de definir una clase nueva. El generador debería producir exactamente un valor. El código hasta yield se ejecutará como el método __enter__(), y el valor obtenido será el valor de retorno del método que se vinculará a la variable en la clausula as (si la hay) de la sentencia “with”. El código después de yield se ejecutará en el método __exit__() . Cualquier excepción lanzada en el bloque será generada por la sentencia yield.

Usando este decorador, nuestro ejemplo de base de datos de la sección anterior podría escribirse como:

from contextlib import contextmanager

@contextmanager
def db_transaction(connection):
    cursor = connection.cursor()
    try:
        yield cursor
    except:
        connection.rollback()
        raise
    else:
        connection.commit()

db = DatabaseConnection()
with db_transaction(db) as cursor:
    ...

El módulo contextlib también tiene una función nested(mgr1, mgr2, ...) que combina varios gestores de contexto para que no necesite escribir sentencias “with” anidadas. En este ejemplo, se utiliza una única sentencia “with” que inicia una transacción de base de datos y adquiere un bloqueo del hilo:

lock = threading.Lock()
with nested (db_transaction(db), lock) as (cursor, locked):
    ...

Por último, la función close() retorna su argumento para que pueda vincularse a una variable, y llama al método .close() del argumento al final del bloque.

import urllib, sys
from contextlib import closing

with closing(urllib.urlopen('http://www.yahoo.com')) as f:
    for line in f:
        sys.stdout.write(line)

Ver también

PEP 343 - La sentencia «with»

PEP escrito por Guido van Rossum y Nick Coghlan; implementado por Mike Bland, Guido van Rossum y Neal Norwitz. El PEP muestra el código generado para una sentencia “with”, que puede ser útil para aprender cómo la sentencia funciona.

La documentación para el módulo contextlib.

PEP 366: Importaciones relativas explícitas desde un módulo principal

El modificador de Python -m permite ejecutar un módulo como un script. Cuando ejecutabas un módulo que estaba ubicado dentro de un paquete, las importaciones relativas no funcionaban correctamente.

La corrección para Python 2.6 agrega un atributo __package__ a los módulos. Cuando este atributo está presente, las importaciones relativas serán relativas al valor de este atributo en lugar del atributo __name__.

Las importaciones de estilo PEP 302 pueden configurar __package__ según sea necesario. El módulo runpy que implementa el modificador -m ahora hace esto, por lo que las importaciones relativas ahora funcionarán correctamente en los scripts que se ejecutan desde el interior de un paquete.

PEP 370: Directorio de site-packages por usuario

Cuando ejecutas Python, la ruta de búsqueda del módulo sys.path generalmente incluye un directorio cuya ruta termina en "site-packages". Este directorio está destinado a contener paquetes instalados localmente disponibles para todos los usuarios que utilizan una máquina o un sitio de instalación en particular.

Python 2.6 introduce una convención para directorios de sitios específicos del usuario. El directorio varía según la plataforma:

• Unix y Mac OS X: ~/.local/

• Windows: %APPDATA%/Python

Dentro de este directorio, habrá subdirectorios específicos de versión, como lib/python2.6/site-packages en Unix/Mac OS y Python26/site-packages en Windows.

Si no le gusta el directorio predeterminado, puede sobrescribirlo mediante una variable de entorno. PYTHONUSERBASE establece el directorio raíz utilizado para todas las versiones de Python que admiten esta función. En Windows, el directorio de datos específicos de la aplicación se puede cambiar configurando la variable de entorno APPDATA. También puede modificar el archivo site.py para su instalación de Python.

La característica se puede desactivar por completo ejecutando Python con la opción -s o seteando la variable de entorno PYTHONNOUSERSITE.

Ver también

PEP 370: Directorio de site-packages por usuario

PEP escrito e implementado por Christian Heimes.

PEP 371: El paquete multiprocessing

El nuevo paquete multiprocessing permite a los programas de Python crear nuevos procesos que realizarán un cálculo y retornaran un resultado al padre. Los procesos padre e hijo pueden comunicarse mediante colas (queues) y tuberías (pipes), sincronizar sus operaciones mediante bloqueos y semáforos, y pueden compartir matrices simples de datos.

El módulo multiprocessing comenzó como una emulación exacta del módulo threading usando procesos en lugar de hilos. Ese objetivo se descartó en el camino a Python 2.6, pero el enfoque general del módulo sigue siendo similar. La clase fundamental es Process, a la que se le pasa un objeto invocable y una colección de argumentos. El método start() establece el invocable ejecutándose en un subproceso, después de lo cual se puede llamar al método is_alive() para verificar si el subproceso aún se está ejecutando y al método join() para esperar al proceso para salir.

Aquí hay un ejemplo simple donde el subproceso calculará un factorial. La función que realiza el cálculo está escrita de forma extraña, por lo que lleva mucho más tiempo cuando el argumento de entrada es un múltiplo de 4.

import time
from multiprocessing import Process, Queue


def factorial(queue, N):
    "Compute a factorial."
    # If N is a multiple of 4, this function will take much longer.
    if (N % 4) == 0:
        time.sleep(.05 * N/4)

    # Calculate the result
    fact = 1L
    for i in range(1, N+1):
        fact = fact * i

    # Put the result on the queue
    queue.put(fact)

if __name__ == '__main__':
    queue = Queue()

    N = 5

    p = Process(target=factorial, args=(queue, N))
    p.start()
    p.join()

    result = queue.get()
    print 'Factorial', N, '=', result

Un Queue se usa para comunicar el resultado del factorial. El objeto Queue se almacena en una variable global. El proceso hijo usará el valor de la variable cuando se creó el hijo; porque es una Queue, padre e hijo pueden usar el objeto para comunicarse. (Si el padre cambiara el valor de la variable global, el valor del hijo no se vería afectado y viceversa).

Otras dos clases, Pool y Manager, proporcionan interfaces de nivel superior. Pool creará un número fijo de procesos de trabajo, y las solicitudes se pueden distribuir a los trabajadores llamando a apply() o apply_async() para agregar una sola solicitud, y map() o map_async() para agregar una serie de solicitudes. El siguiente código usa Pool para distribuir las solicitudes en 5 procesos de trabajo y recuperar una lista de resultados:

from multiprocessing import Pool

def factorial(N, dictionary):
    "Compute a factorial."
    ...
p = Pool(5)
result = p.map(factorial, range(1, 1000, 10))
for v in result:
    print v

Esto produce la siguiente salida:

1
39916800
51090942171709440000
8222838654177922817725562880000000
33452526613163807108170062053440751665152000000000
...

La otra interfaz de alto nivel, la clase Manager, crea un proceso de servidor separado que puede contener copias maestras de las estructuras de datos de Python. Luego, otros procesos pueden acceder y modificar estas estructuras de datos utilizando objetos proxy. El siguiente ejemplo crea un diccionario compartido llamando al método dict(); los procesos de trabajo luego insertan valores en el diccionario. (El bloqueo no se realiza automáticamente, lo cual no importa en este ejemplo. Los métodos de Manager también incluyen Lock(), RLock(), y Semaphore() para crear bloqueos compartidos.)

import time
from multiprocessing import Pool, Manager

def factorial(N, dictionary):
    "Compute a factorial."
    # Calculate the result
    fact = 1L
    for i in range(1, N+1):
        fact = fact * i

    # Store result in dictionary
    dictionary[N] = fact

if __name__ == '__main__':
    p = Pool(5)
    mgr = Manager()
    d = mgr.dict()         # Create shared dictionary

    # Run tasks using the pool
    for N in range(1, 1000, 10):
        p.apply_async(factorial, (N, d))

    # Mark pool as closed -- no more tasks can be added.
    p.close()

    # Wait for tasks to exit
    p.join()

    # Output results
    for k, v in sorted(d.items()):
        print k, v

Esto producirá la salida:

1 1
11 39916800
21 51090942171709440000
31 8222838654177922817725562880000000
41 33452526613163807108170062053440751665152000000000
51 15511187532873822802242430164693032110632597200169861120000...

Ver también

La documentación del módulo multiprocessing.

PEP 371 - Adición del paquete de multiprocesamiento

PEP escrito por Jesse Noller y Richard Oudkerk; implementado por Richard Oudkerk y Jesse Noller.

PEP 3101: Formateo avanzado de cadena de caracteres

En Python 3.0, el operador % se complementa con un método de formato de cadena de caracteres más potente, format(). La compatibilidad con el método str.format() se ha actualizado a Python 2.6.

En 2.6, las cadenas de caracteres de 8 bits y Unicode tienen un método .format() que trata la cadena como una plantilla y toma los argumentos a formatear. La plantilla de formato utiliza llaves ({, }) como caracteres especiales:

>>> # Substitute positional argument 0 into the string.
>>> "User ID: {0}".format("root")
'User ID: root'
>>> # Use the named keyword arguments
>>> "User ID: {uid}   Last seen: {last_login}".format(
...    uid="root",
...    last_login = "5 Mar 2008 07:20")
'User ID: root   Last seen: 5 Mar 2008 07:20'

Las llaves se pueden escapar duplicándose:

>>> "Empty dict: {{}}".format()
"Empty dict: {}"

Los nombres de campo pueden ser números enteros que indican argumentos posicionales, como {0}, {1}, etc. o nombres de argumentos de palabras clave. También puede proporcionar nombres de campos compuestos que lean atributos o accedan a claves de diccionario:

>>> import sys
>>> print 'Platform: {0.platform}\nPython version: {0.version}'.format(sys)
Platform: darwin
Python version: 2.6a1+ (trunk:61261M, Mar  5 2008, 20:29:41)
[GCC 4.0.1 (Apple Computer, Inc. build 5367)]'

>>> import mimetypes
>>> 'Content-type: {0[.mp4]}'.format(mimetypes.types_map)
'Content-type: video/mp4'

Tenga en cuenta que cuando utilice una notación de estilo diccionario como [.mp4], no es necesario poner comillas alrededor de la cadena; se buscará el valor usando .mp4 como clave. Las cadenas de caracteres que comienzan con un número se convertirán en entero. No puede escribir expresiones más complicadas dentro de una cadena de formato.

Hasta ahora hemos mostrado cómo especificar qué campo sustituir en la cadena resultante. El formato preciso utilizado también se puede controlar agregando dos puntos seguidos de un especificador de formato. Por ejemplo:

>>> # Field 0: left justify, pad to 15 characters
>>> # Field 1: right justify, pad to 6 characters
>>> fmt = '{0:15} ${1:>6}'
>>> fmt.format('Registration', 35)
'Registration    $    35'
>>> fmt.format('Tutorial', 50)
'Tutorial        $    50'
>>> fmt.format('Banquet', 125)
'Banquet         $   125'

Los especificadores de formato pueden hacer referencia a otros campos a través del anidamiento:

>>> fmt = '{0:{1}}'
>>> width = 15
>>> fmt.format('Invoice #1234', width)
'Invoice #1234  '
>>> width = 35
>>> fmt.format('Invoice #1234', width)
'Invoice #1234                      '

Se puede especificar la alineación de un campo dentro del ancho deseado:

Carácter	Efecto
< (por defecto)	Alinear a la izquierda
>	Alinear a la derecha
^	Centrado
=	(Solo para tipos numéricos) Relleno después del signo.

Los especificadores de formato también pueden incluir un tipo de presentación, que controla cómo se formatea el valor. Por ejemplo, los números de punto flotante pueden formatearse como un número general o en notación exponencial:

>>> '{0:g}'.format(3.75)
'3.75'
>>> '{0:e}'.format(3.75)
'3.750000e+00'

Hay una variedad de tipos de presentación disponibles. Consulte la documentación 2.6 para obtener una lista completa; aquí hay un ejemplo:

b	Binario. Emite el número en base 2.
c	Character. Converts the integer to the corresponding Unicode character before printing.
d	Decimal Integer. Outputs the number in base 10.
o	Octal format. Outputs the number in base 8.
x	Hex format. Outputs the number in base 16, using lower-case letters for the digits above 9.
e	Exponent notation. Prints the number in scientific notation using the letter “e” to indicate the exponent.
g	General format. This prints the number as a fixed-point number, unless the number is too large, in which case it switches to “e” exponent notation.
n	Number. This is the same as “g” (for floats) or “d” (for integers), except that it uses the current locale setting to insert the appropriate number separator characters.
%	Percentage. Multiplies the number by 100 and displays in fixed (“f”) format, followed by a percent sign.

Classes and types can define a __format__() method to control how they’re formatted. It receives a single argument, the format specifier:

def __format__(self, format_spec):
    if isinstance(format_spec, unicode):
        return unicode(str(self))
    else:
        return str(self)

There’s also a format() builtin that will format a single value. It calls the type’s __format__() method with the provided specifier:

>>> format(75.6564, '.2f')
'75.66'

Ver también

Sintaxis de formateo de cadena

The reference documentation for format fields.

PEP 3101 - Advanced String Formatting

PEP written by Talin. Implemented by Eric Smith.

PEP 3105: print As a Function

The print statement becomes the print() function in Python 3.0. Making print() a function makes it possible to replace the function by doing def print(...) or importing a new function from somewhere else.

Python 2.6 has a __future__ import that removes print as language syntax, letting you use the functional form instead. For example:

>>> from __future__ import print_function
>>> print('# of entries', len(dictionary), file=sys.stderr)

The signature of the new function is:

def print(*args, sep=' ', end='\n', file=None)

The parameters are:

• args: positional arguments whose values will be printed out.

• sep: the separator, which will be printed between arguments.

• end: the ending text, which will be printed after all of the arguments have been output.

• file: the file object to which the output will be sent.

Ver también

PEP 3105 - Make print a function

PEP written by Georg Brandl.

PEP 3110: Exception-Handling Changes

One error that Python programmers occasionally make is writing the following code:

try:
    ...
except TypeError, ValueError:  # Wrong!
    ...

The author is probably trying to catch both TypeError and ValueError exceptions, but this code actually does something different: it will catch TypeError and bind the resulting exception object to the local name "ValueError". The ValueError exception will not be caught at all. The correct code specifies a tuple of exceptions:

try:
    ...
except (TypeError, ValueError):
    ...

This error happens because the use of the comma here is ambiguous: does it indicate two different nodes in the parse tree, or a single node that’s a tuple?

Python 3.0 makes this unambiguous by replacing the comma with the word «as». To catch an exception and store the exception object in the variable exc, you must write:

try:
    ...
except TypeError as exc:
    ...

Python 3.0 will only support the use of «as», and therefore interprets the first example as catching two different exceptions. Python 2.6 supports both the comma and «as», so existing code will continue to work. We therefore suggest using «as» when writing new Python code that will only be executed with 2.6.

Ver también

PEP 3110 - Catching Exceptions in Python 3000

PEP written and implemented by Collin Winter.

PEP 3112: Byte Literals

Python 3.0 adopts Unicode as the language’s fundamental string type and denotes 8-bit literals differently, either as b'string' or using a bytes constructor. For future compatibility, Python 2.6 adds bytes as a synonym for the str type, and it also supports the b'' notation.

The 2.6 str differs from 3.0’s bytes type in various ways; most notably, the constructor is completely different. In 3.0, bytes([65, 66, 67]) is 3 elements long, containing the bytes representing ABC; in 2.6, bytes([65, 66, 67]) returns the 12-byte string representing the str() of the list.

The primary use of bytes in 2.6 will be to write tests of object type such as isinstance(x, bytes). This will help the 2to3 converter, which can’t tell whether 2.x code intends strings to contain either characters or 8-bit bytes; you can now use either bytes or str to represent your intention exactly, and the resulting code will also be correct in Python 3.0.

There’s also a __future__ import that causes all string literals to become Unicode strings. This means that \u escape sequences can be used to include Unicode characters:

from __future__ import unicode_literals

s = ('\u751f\u3080\u304e\u3000\u751f\u3054'
     '\u3081\u3000\u751f\u305f\u307e\u3054')

print len(s)               # 12 Unicode characters

At the C level, Python 3.0 will rename the existing 8-bit string type, called PyStringObject in Python 2.x, to PyBytesObject. Python 2.6 uses #define to support using the names PyBytesObject(), PyBytes_Check(), PyBytes_FromStringAndSize(), and all the other functions and macros used with strings.

Instances of the bytes type are immutable just as strings are. A new bytearray type stores a mutable sequence of bytes:

>>> bytearray([65, 66, 67])
bytearray(b'ABC')
>>> b = bytearray(u'\u21ef\u3244', 'utf-8')
>>> b
bytearray(b'\xe2\x87\xaf\xe3\x89\x84')
>>> b[0] = '\xe3'
>>> b
bytearray(b'\xe3\x87\xaf\xe3\x89\x84')
>>> unicode(str(b), 'utf-8')
u'\u31ef \u3244'

Byte arrays support most of the methods of string types, such as startswith()/endswith(), find()/rfind(), and some of the methods of lists, such as append(), pop(), and reverse().

>>> b = bytearray('ABC')
>>> b.append('d')
>>> b.append(ord('e'))
>>> b
bytearray(b'ABCde')

There’s also a corresponding C API, with PyByteArray_FromObject(), PyByteArray_FromStringAndSize(), and various other functions.

Ver también

PEP 3112 - Bytes literals in Python 3000

PEP written by Jason Orendorff; backported to 2.6 by Christian Heimes.

PEP 3116: New I/O Library

Python’s built-in file objects support a number of methods, but file-like objects don’t necessarily support all of them. Objects that imitate files usually support read() and write(), but they may not support readline(), for example. Python 3.0 introduces a layered I/O library in the io module that separates buffering and text-handling features from the fundamental read and write operations.

There are three levels of abstract base classes provided by the io module:

• RawIOBase defines raw I/O operations: read(), readinto(), write(), seek(), tell(), truncate(), and close(). Most of the methods of this class will often map to a single system call. There are also readable(), writable(), and seekable() methods for determining what operations a given object will allow.

  Python 3.0 has concrete implementations of this class for files and sockets, but Python 2.6 hasn’t restructured its file and socket objects in this way.

• BufferedIOBase is an abstract base class that buffers data in memory to reduce the number of system calls used, making I/O processing more efficient. It supports all of the methods of RawIOBase, and adds a raw attribute holding the underlying raw object.

  There are five concrete classes implementing this ABC. BufferedWriter and BufferedReader are for objects that support write-only or read-only usage that have a seek() method for random access. BufferedRandom objects support read and write access upon the same underlying stream, and BufferedRWPair is for objects such as TTYs that have both read and write operations acting upon unconnected streams of data. The BytesIO class supports reading, writing, and seeking over an in-memory buffer.

• TextIOBase: Provides functions for reading and writing strings (remember, strings will be Unicode in Python 3.0), and supporting universal newlines. TextIOBase defines the readline() method and supports iteration upon objects.

  There are two concrete implementations. TextIOWrapper wraps a buffered I/O object, supporting all of the methods for text I/O and adding a buffer attribute for access to the underlying object. StringIO simply buffers everything in memory without ever writing anything to disk.

  (In Python 2.6, io.StringIO is implemented in pure Python, so it’s pretty slow. You should therefore stick with the existing StringIO module or cStringIO for now. At some point Python 3.0’s io module will be rewritten into C for speed, and perhaps the C implementation will be backported to the 2.x releases.)

In Python 2.6, the underlying implementations haven’t been restructured to build on top of the io module’s classes. The module is being provided to make it easier to write code that’s forward-compatible with 3.0, and to save developers the effort of writing their own implementations of buffering and text I/O.

Ver también

PEP 3116 - New I/O

PEP written by Daniel Stutzbach, Mike Verdone, and Guido van Rossum. Code by Guido van Rossum, Georg Brandl, Walter Doerwald, Jeremy Hylton, Martin von Löwis, Tony Lownds, and others.

PEP 3118: Revised Buffer Protocol

The buffer protocol is a C-level API that lets Python types exchange pointers into their internal representations. A memory-mapped file can be viewed as a buffer of characters, for example, and this lets another module such as re treat memory-mapped files as a string of characters to be searched.

The primary users of the buffer protocol are numeric-processing packages such as NumPy, which expose the internal representation of arrays so that callers can write data directly into an array instead of going through a slower API. This PEP updates the buffer protocol in light of experience from NumPy development, adding a number of new features such as indicating the shape of an array or locking a memory region.

The most important new C API function is PyObject_GetBuffer(PyObject *obj, Py_buffer *view, int flags), which takes an object and a set of flags, and fills in the Py_buffer structure with information about the object’s memory representation. Objects can use this operation to lock memory in place while an external caller could be modifying the contents, so there’s a corresponding PyBuffer_Release(Py_buffer *view) to indicate that the external caller is done.

The flags argument to PyObject_GetBuffer() specifies constraints upon the memory returned. Some examples are:

• PyBUF_WRITABLE indicates that the memory must be writable.

• PyBUF_LOCK requests a read-only or exclusive lock on the memory.

• PyBUF_C_CONTIGUOUS and PyBUF_F_CONTIGUOUS requests a C-contiguous (last dimension varies the fastest) or Fortran-contiguous (first dimension varies the fastest) array layout.

Two new argument codes for PyArg_ParseTuple(), s* and z*, return locked buffer objects for a parameter.

Ver también

PEP 3118 - Revising the buffer protocol

PEP written by Travis Oliphant and Carl Banks; implemented by Travis Oliphant.

PEP 3119: Abstract Base Classes

Some object-oriented languages such as Java support interfaces, declaring that a class has a given set of methods or supports a given access protocol. Abstract Base Classes (or ABCs) are an equivalent feature for Python. The ABC support consists of an abc module containing a metaclass called ABCMeta, special handling of this metaclass by the isinstance() and issubclass() builtins, and a collection of basic ABCs that the Python developers think will be widely useful. Future versions of Python will probably add more ABCs.

Let’s say you have a particular class and wish to know whether it supports dictionary-style access. The phrase «dictionary-style» is vague, however. It probably means that accessing items with obj[1] works. Does it imply that setting items with obj[2] = value works? Or that the object will have keys(), values(), and items() methods? What about the iterative variants such as iterkeys()? copy() and update()? Iterating over the object with iter()?

The Python 2.6 collections module includes a number of different ABCs that represent these distinctions. Iterable indicates that a class defines __iter__(), and Container means the class defines a __contains__() method and therefore supports x in y expressions. The basic dictionary interface of getting items, setting items, and keys(), values(), and items(), is defined by the MutableMapping ABC.

You can derive your own classes from a particular ABC to indicate they support that ABC’s interface:

import collections

class Storage(collections.MutableMapping):
    ...

Alternatively, you could write the class without deriving from the desired ABC and instead register the class by calling the ABC’s register() method:

import collections

class Storage:
    ...

collections.MutableMapping.register(Storage)

For classes that you write, deriving from the ABC is probably clearer. The register() method is useful when you’ve written a new ABC that can describe an existing type or class, or if you want to declare that some third-party class implements an ABC. For example, if you defined a PrintableType ABC, it’s legal to do:

# Register Python's types
PrintableType.register(int)
PrintableType.register(float)
PrintableType.register(str)

Classes should obey the semantics specified by an ABC, but Python can’t check this; it’s up to the class author to understand the ABC’s requirements and to implement the code accordingly.

To check whether an object supports a particular interface, you can now write:

def func(d):
    if not isinstance(d, collections.MutableMapping):
        raise ValueError("Mapping object expected, not %r" % d)

Don’t feel that you must now begin writing lots of checks as in the above example. Python has a strong tradition of duck-typing, where explicit type-checking is never done and code simply calls methods on an object, trusting that those methods will be there and raising an exception if they aren’t. Be judicious in checking for ABCs and only do it where it’s absolutely necessary.

You can write your own ABCs by using abc.ABCMeta as the metaclass in a class definition:

from abc import ABCMeta, abstractmethod

class Drawable():
    __metaclass__ = ABCMeta

    @abstractmethod
    def draw(self, x, y, scale=1.0):
        pass

    def draw_doubled(self, x, y):
        self.draw(x, y, scale=2.0)


class Square(Drawable):
    def draw(self, x, y, scale):
        ...

In the Drawable ABC above, the draw_doubled() method renders the object at twice its size and can be implemented in terms of other methods described in Drawable. Classes implementing this ABC therefore don’t need to provide their own implementation of draw_doubled(), though they can do so. An implementation of draw() is necessary, though; the ABC can’t provide a useful generic implementation.

You can apply the @abstractmethod decorator to methods such as draw() that must be implemented; Python will then raise an exception for classes that don’t define the method. Note that the exception is only raised when you actually try to create an instance of a subclass lacking the method:

>>> class Circle(Drawable):
...     pass
...
>>> c = Circle()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class Circle with abstract methods draw
>>>

Abstract data attributes can be declared using the @abstractproperty decorator:

from abc import abstractproperty
...

@abstractproperty
def readonly(self):
   return self._x

Subclasses must then define a readonly() property.

Ver también

PEP 3119 - Introducing Abstract Base Classes

PEP written by Guido van Rossum and Talin. Implemented by Guido van Rossum. Backported to 2.6 by Benjamin Aranguren, with Alex Martelli.

PEP 3127: Integer Literal Support and Syntax

Python 3.0 changes the syntax for octal (base-8) integer literals, prefixing them with «0o» or «0O» instead of a leading zero, and adds support for binary (base-2) integer literals, signalled by a «0b» or «0B» prefix.

Python 2.6 doesn’t drop support for a leading 0 signalling an octal number, but it does add support for «0o» and «0b»:

>>> 0o21, 2*8 + 1
(17, 17)
>>> 0b101111
47

The oct() builtin still returns numbers prefixed with a leading zero, and a new bin() builtin returns the binary representation for a number:

>>> oct(42)
'052'
>>> future_builtins.oct(42)
'0o52'
>>> bin(173)
'0b10101101'

The int() and long() builtins will now accept the «0o» and «0b» prefixes when base-8 or base-2 are requested, or when the base argument is zero (signalling that the base used should be determined from the string):

>>> int ('0o52', 0)
42
>>> int('1101', 2)
13
>>> int('0b1101', 2)
13
>>> int('0b1101', 0)
13

Ver también

PEP 3127 - Integer Literal Support and Syntax

PEP written by Patrick Maupin; backported to 2.6 by Eric Smith.

PEP 3129: Class Decorators

Decorators have been extended from functions to classes. It’s now legal to write:

@foo
@bar
class A:
  pass

This is equivalent to:

class A:
  pass

A = foo(bar(A))

Ver también

PEP 3129 - Class Decorators

PEP written by Collin Winter.

PEP 3141: A Type Hierarchy for Numbers

Python 3.0 adds several abstract base classes for numeric types inspired by Scheme’s numeric tower. These classes were backported to 2.6 as the numbers module.

The most general ABC is Number. It defines no operations at all, and only exists to allow checking if an object is a number by doing isinstance(obj, Number).

Complex is a subclass of Number. Complex numbers can undergo the basic operations of addition, subtraction, multiplication, division, and exponentiation, and you can retrieve the real and imaginary parts and obtain a number’s conjugate. Python’s built-in complex type is an implementation of Complex.

Real further derives from Complex, and adds operations that only work on real numbers: floor(), trunc(), rounding, taking the remainder mod N, floor division, and comparisons.

Rational numbers derive from Real, have numerator and denominator properties, and can be converted to floats. Python 2.6 adds a simple rational-number class, Fraction, in the fractions module. (It’s called Fraction instead of Rational to avoid a name clash with numbers.Rational.)

Integral numbers derive from Rational, and can be shifted left and right with << and >>, combined using bitwise operations such as & and |, and can be used as array indexes and slice boundaries.

In Python 3.0, the PEP slightly redefines the existing builtins round(), math.floor(), math.ceil(), and adds a new one, math.trunc(), that’s been backported to Python 2.6. math.trunc() rounds toward zero, returning the closest Integral that’s between the function’s argument and zero.

Ver también

PEP 3141 - A Type Hierarchy for Numbers

PEP written by Jeffrey Yasskin.

Scheme’s numerical tower, from the Guile manual.

Scheme’s number datatypes from the R5RS Scheme specification.

The fractions Module

To fill out the hierarchy of numeric types, the fractions module provides a rational-number class. Rational numbers store their values as a numerator and denominator forming a fraction, and can exactly represent numbers such as 2/3 that floating-point numbers can only approximate.

The Fraction constructor takes two Integral values that will be the numerator and denominator of the resulting fraction.

>>> from fractions import Fraction
>>> a = Fraction(2, 3)
>>> b = Fraction(2, 5)
>>> float(a), float(b)
(0.66666666666666663, 0.40000000000000002)
>>> a+b
Fraction(16, 15)
>>> a/b
Fraction(5, 3)

For converting floating-point numbers to rationals, the float type now has an as_integer_ratio() method that returns the numerator and denominator for a fraction that evaluates to the same floating-point value:

>>> (2.5) .as_integer_ratio()
(5, 2)
>>> (3.1415) .as_integer_ratio()
(7074029114692207L, 2251799813685248L)
>>> (1./3) .as_integer_ratio()
(6004799503160661L, 18014398509481984L)

Note that values that can only be approximated by floating-point numbers, such as 1./3, are not simplified to the number being approximated; the fraction attempts to match the floating-point value exactly.

The fractions module is based upon an implementation by Sjoerd Mullender that was in Python’s Demo/classes/ directory for a long time. This implementation was significantly updated by Jeffrey Yasskin.

Other Language Changes

Some smaller changes made to the core Python language are:

• Directories and zip archives containing a __main__.py file can now be executed directly by passing their name to the interpreter. The directory or zip archive is automatically inserted as the first entry in sys.path. (Suggestion and initial patch by Andy Chu, subsequently revised by Phillip J. Eby and Nick Coghlan; bpo-1739468.)

• The hasattr() function was catching and ignoring all errors, under the assumption that they meant a __getattr__() method was failing somehow and the return value of hasattr() would therefore be False. This logic shouldn’t be applied to KeyboardInterrupt and SystemExit, however; Python 2.6 will no longer discard such exceptions when hasattr() encounters them. (Fixed by Benjamin Peterson; bpo-2196.)

• When calling a function using the ** syntax to provide keyword arguments, you are no longer required to use a Python dictionary; any mapping will now work:

  >>> def f(**kw):
  ...    print sorted(kw)
  ...
  >>> ud=UserDict.UserDict()
  >>> ud['a'] = 1
  >>> ud['b'] = 'string'
  >>> f(**ud)
  ['a', 'b']
  

  (Contributed by Alexander Belopolsky; bpo-1686487.)

  It’s also become legal to provide keyword arguments after a *args argument to a function call.

  >>> def f(*args, **kw):
  ...     print args, kw
  ...
  >>> f(1,2,3, *(4,5,6), keyword=13)
  (1, 2, 3, 4, 5, 6) {'keyword': 13}
  

  Previously this would have been a syntax error. (Contributed by Amaury Forgeot d’Arc; bpo-3473.)

• A new builtin, next(iterator, [default]) returns the next item from the specified iterator. If the default argument is supplied, it will be returned if iterator has been exhausted; otherwise, the StopIteration exception will be raised. (Backported in bpo-2719.)

• Tuples now have index() and count() methods matching the list type’s index() and count() methods:

  >>> t = (0,1,2,3,4,0,1,2)
  >>> t.index(3)
  3
  >>> t.count(0)
  2
  

  (Contributed by Raymond Hettinger)

• The built-in types now have improved support for extended slicing syntax, accepting various combinations of (start, stop, step). Previously, the support was partial and certain corner cases wouldn’t work. (Implemented by Thomas Wouters.)

• Properties now have three attributes, getter, setter and deleter, that are decorators providing useful shortcuts for adding a getter, setter or deleter function to an existing property. You would use them like this:

  class C(object):
      @property
      def x(self):
          return self._x
  
      @x.setter
      def x(self, value):
          self._x = value
  
      @x.deleter
      def x(self):
          del self._x
  
  class D(C):
      @C.x.getter
      def x(self):
          return self._x * 2
  
      @x.setter
      def x(self, value):
          self._x = value / 2
  

• Several methods of the built-in set types now accept multiple iterables: intersection(), intersection_update(), union(), update(), difference() and difference_update().

  >>> s=set('1234567890')
  >>> s.intersection('abc123', 'cdf246')  # Intersection between all inputs
  set(['2'])
  >>> s.difference('246', '789')
  set(['1', '0', '3', '5'])
  

  (Contributed by Raymond Hettinger.)

• Many floating-point features were added. The float() function will now turn the string nan into an IEEE 754 Not A Number value, and +inf and -inf into positive or negative infinity. This works on any platform with IEEE 754 semantics. (Contributed by Christian Heimes; bpo-1635.)

  Other functions in the math module, isinf() and isnan(), return true if their floating-point argument is infinite or Not A Number. (bpo-1640)

  Conversion functions were added to convert floating-point numbers into hexadecimal strings (bpo-3008). These functions convert floats to and from a string representation without introducing rounding errors from the conversion between decimal and binary. Floats have a hex() method that returns a string representation, and the float.fromhex() method converts a string back into a number:

  >>> a = 3.75
  >>> a.hex()
  '0x1.e000000000000p+1'
  >>> float.fromhex('0x1.e000000000000p+1')
  3.75
  >>> b=1./3
  >>> b.hex()
  '0x1.5555555555555p-2'
  

• A numerical nicety: when creating a complex number from two floats on systems that support signed zeros (-0 and +0), the complex() constructor will now preserve the sign of the zero. (Fixed by Mark T. Dickinson; bpo-1507.)

• Classes that inherit a __hash__() method from a parent class can set __hash__ = None to indicate that the class isn’t hashable. This will make hash(obj) raise a TypeError and the class will not be indicated as implementing the Hashable ABC.

  You should do this when you’ve defined a __cmp__() or __eq__() method that compares objects by their value rather than by identity. All objects have a default hash method that uses id(obj) as the hash value. There’s no tidy way to remove the __hash__() method inherited from a parent class, so assigning None was implemented as an override. At the C level, extensions can set tp_hash to PyObject_HashNotImplemented(). (Fixed by Nick Coghlan and Amaury Forgeot d’Arc; bpo-2235.)

• The GeneratorExit exception now subclasses BaseException instead of Exception. This means that an exception handler that does except Exception: will not inadvertently catch GeneratorExit. (Contributed by Chad Austin; bpo-1537.)

• Generator objects now have a gi_code attribute that refers to the original code object backing the generator. (Contributed by Collin Winter; bpo-1473257.)

• The compile() built-in function now accepts keyword arguments as well as positional parameters. (Contributed by Thomas Wouters; bpo-1444529.)

• The complex() constructor now accepts strings containing parenthesized complex numbers, meaning that complex(repr(cplx)) will now round-trip values. For example, complex('(3+4j)') now returns the value (3+4j). (bpo-1491866)

• The string translate() method now accepts None as the translation table parameter, which is treated as the identity transformation. This makes it easier to carry out operations that only delete characters. (Contributed by Bengt Richter and implemented by Raymond Hettinger; bpo-1193128.)

• The built-in dir() function now checks for a __dir__() method on the objects it receives. This method must return a list of strings containing the names of valid attributes for the object, and lets the object control the value that dir() produces. Objects that have __getattr__() or __getattribute__() methods can use this to advertise pseudo-attributes they will honor. (bpo-1591665)

• Instance method objects have new attributes for the object and function comprising the method; the new synonym for im_self is __self__, and im_func is also available as __func__. The old names are still supported in Python 2.6, but are gone in 3.0.

• An obscure change: when you use the locals() function inside a class statement, the resulting dictionary no longer returns free variables. (Free variables, in this case, are variables referenced in the class statement that aren’t attributes of the class.)

Optimizaciones

• The warnings module has been rewritten in C. This makes it possible to invoke warnings from the parser, and may also make the interpreter’s startup faster. (Contributed by Neal Norwitz and Brett Cannon; bpo-1631171.)

• Type objects now have a cache of methods that can reduce the work required to find the correct method implementation for a particular class; once cached, the interpreter doesn’t need to traverse base classes to figure out the right method to call. The cache is cleared if a base class or the class itself is modified, so the cache should remain correct even in the face of Python’s dynamic nature. (Original optimization implemented by Armin Rigo, updated for Python 2.6 by Kevin Jacobs; bpo-1700288.)

  By default, this change is only applied to types that are included with the Python core. Extension modules may not necessarily be compatible with this cache, so they must explicitly add Py_TPFLAGS_HAVE_VERSION_TAG to the module’s tp_flags field to enable the method cache. (To be compatible with the method cache, the extension module’s code must not directly access and modify the tp_dict member of any of the types it implements. Most modules don’t do this, but it’s impossible for the Python interpreter to determine that. See bpo-1878 for some discussion.)

• Function calls that use keyword arguments are significantly faster by doing a quick pointer comparison, usually saving the time of a full string comparison. (Contributed by Raymond Hettinger, after an initial implementation by Antoine Pitrou; bpo-1819.)

• All of the functions in the struct module have been rewritten in C, thanks to work at the Need For Speed sprint. (Contributed by Raymond Hettinger.)

• Some of the standard built-in types now set a bit in their type objects. This speeds up checking whether an object is a subclass of one of these types. (Contributed by Neal Norwitz.)

• Unicode strings now use faster code for detecting whitespace and line breaks; this speeds up the split() method by about 25% and splitlines() by 35%. (Contributed by Antoine Pitrou.) Memory usage is reduced by using pymalloc for the Unicode string’s data.

• The with statement now stores the __exit__() method on the stack, producing a small speedup. (Implemented by Jeffrey Yasskin.)

• To reduce memory usage, the garbage collector will now clear internal free lists when garbage-collecting the highest generation of objects. This may return memory to the operating system sooner.

Interpreter Changes

Two command-line options have been reserved for use by other Python implementations. The -J switch has been reserved for use by Jython for Jython-specific options, such as switches that are passed to the underlying JVM. -X has been reserved for options specific to a particular implementation of Python such as CPython, Jython, or IronPython. If either option is used with Python 2.6, the interpreter will report that the option isn’t currently used.

Python can now be prevented from writing .pyc or .pyo files by supplying the -B switch to the Python interpreter, or by setting the PYTHONDONTWRITEBYTECODE environment variable before running the interpreter. This setting is available to Python programs as the sys.dont_write_bytecode variable, and Python code can change the value to modify the interpreter’s behaviour. (Contributed by Neal Norwitz and Georg Brandl.)

The encoding used for standard input, output, and standard error can be specified by setting the PYTHONIOENCODING environment variable before running the interpreter. The value should be a string in the form <encoding> or <encoding>:<errorhandler>. The encoding part specifies the encoding’s name, e.g. utf-8 or latin-1; the optional errorhandler part specifies what to do with characters that can’t be handled by the encoding, and should be one of «error», «ignore», or «replace». (Contributed by Martin von Löwis.)

New and Improved Modules

As in every release, Python’s standard library received a number of enhancements and bug fixes. Here’s a partial list of the most notable changes, sorted alphabetically by module name. Consult the Misc/NEWS file in the source tree for a more complete list of changes, or look through the Subversion logs for all the details.

• The asyncore and asynchat modules are being actively maintained again, and a number of patches and bugfixes were applied. (Maintained by Josiah Carlson; see bpo-1736190 for one patch.)

• The bsddb module also has a new maintainer, Jesús Cea Avión, and the package is now available as a standalone package. The web page for the package is www.jcea.es/programacion/pybsddb.htm. The plan is to remove the package from the standard library in Python 3.0, because its pace of releases is much more frequent than Python’s.

  The bsddb.dbshelve module now uses the highest pickling protocol available, instead of restricting itself to protocol 1. (Contributed by W. Barnes.)

• The cgi module will now read variables from the query string of an HTTP POST request. This makes it possible to use form actions with URLs that include query strings such as «/cgi-bin/add.py?category=1». (Contributed by Alexandre Fiori and Nubis; bpo-1817.)

  The parse_qs() and parse_qsl() functions have been relocated from the cgi module to the urlparse module. The versions still available in the cgi module will trigger PendingDeprecationWarning messages in 2.6 (bpo-600362).

• The cmath module underwent extensive revision, contributed by Mark Dickinson and Christian Heimes. Five new functions were added:

  • polar() converts a complex number to polar form, returning the modulus and argument of the complex number.

  • rect() does the opposite, turning a modulus, argument pair back into the corresponding complex number.

  • phase() returns the argument (also called the angle) of a complex number.

  • isnan() returns True if either the real or imaginary part of its argument is a NaN.

  • isinf() returns True if either the real or imaginary part of its argument is infinite.

  The revisions also improved the numerical soundness of the cmath module. For all functions, the real and imaginary parts of the results are accurate to within a few units of least precision (ulps) whenever possible. See bpo-1381 for the details. The branch cuts for asinh(), atanh(): and atan() have also been corrected.

  The tests for the module have been greatly expanded; nearly 2000 new test cases exercise the algebraic functions.

  On IEEE 754 platforms, the cmath module now handles IEEE 754 special values and floating-point exceptions in a manner consistent with Annex “G” of the C99 standard.

• A new data type in the collections module: namedtuple(typename, fieldnames) is a factory function that creates subclasses of the standard tuple whose fields are accessible by name as well as index. For example:

  >>> var_type = collections.namedtuple('variable',
  ...             'id name type size')
  >>> # Names are separated by spaces or commas.
  >>> # 'id, name, type, size' would also work.
  >>> var_type._fields
  ('id', 'name', 'type', 'size')
  
  >>> var = var_type(1, 'frequency', 'int', 4)
  >>> print var[0], var.id    # Equivalent
  1 1
  >>> print var[2], var.type  # Equivalent
  int int
  >>> var._asdict()
  {'size': 4, 'type': 'int', 'id': 1, 'name': 'frequency'}
  >>> v2 = var._replace(name='amplitude')
  >>> v2
  variable(id=1, name='amplitude', type='int', size=4)
  

  Several places in the standard library that returned tuples have been modified to return namedtuple instances. For example, the Decimal.as_tuple() method now returns a named tuple with sign, digits, and exponent fields.

  (Contributed by Raymond Hettinger.)

• Another change to the collections module is that the deque type now supports an optional maxlen parameter; if supplied, the deque’s size will be restricted to no more than maxlen items. Adding more items to a full deque causes old items to be discarded.

  >>> from collections import deque
  >>> dq=deque(maxlen=3)
  >>> dq
  deque([], maxlen=3)
  >>> dq.append(1); dq.append(2); dq.append(3)
  >>> dq
  deque([1, 2, 3], maxlen=3)
  >>> dq.append(4)
  >>> dq
  deque([2, 3, 4], maxlen=3)
  

  (Contributed by Raymond Hettinger.)

• The Cookie module’s Morsel objects now support an httponly attribute. In some browsers. cookies with this attribute set cannot be accessed or manipulated by JavaScript code. (Contributed by Arvin Schnell; bpo-1638033.)

• A new window method in the curses module, chgat(), changes the display attributes for a certain number of characters on a single line. (Contributed by Fabian Kreutz.)

  # Boldface text starting at y=0,x=21
  # and affecting the rest of the line.
  stdscr.chgat(0, 21, curses.A_BOLD)
  

  The Textbox class in the curses.textpad module now supports editing in insert mode as well as overwrite mode. Insert mode is enabled by supplying a true value for the insert_mode parameter when creating the Textbox instance.

• The datetime module’s strftime() methods now support a %f format code that expands to the number of microseconds in the object, zero-padded on the left to six places. (Contributed by Skip Montanaro; bpo-1158.)

• The decimal module was updated to version 1.66 of the General Decimal Specification. New features include some methods for some basic mathematical functions such as exp() and log10():

  >>> Decimal(1).exp()
  Decimal("2.718281828459045235360287471")
  >>> Decimal("2.7182818").ln()
  Decimal("0.9999999895305022877376682436")
  >>> Decimal(1000).log10()
  Decimal("3")
  

  The as_tuple() method of Decimal objects now returns a named tuple with sign, digits, and exponent fields.

  (Implemented by Facundo Batista and Mark Dickinson. Named tuple support added by Raymond Hettinger.)

• The difflib module’s SequenceMatcher class now returns named tuples representing matches, with a, b, and size attributes. (Contributed by Raymond Hettinger.)

• An optional timeout parameter, specifying a timeout measured in seconds, was added to the ftplib.FTP class constructor as well as the connect() method. (Added by Facundo Batista.) Also, the FTP class’s storbinary() and storlines() now take an optional callback parameter that will be called with each block of data after the data has been sent. (Contributed by Phil Schwartz; bpo-1221598.)

• The reduce() built-in function is also available in the functools module. In Python 3.0, the builtin has been dropped and reduce() is only available from functools; currently there are no plans to drop the builtin in the 2.x series. (Patched by Christian Heimes; bpo-1739906.)

• When possible, the getpass module will now use /dev/tty to print a prompt message and read the password, falling back to standard error and standard input. If the password may be echoed to the terminal, a warning is printed before the prompt is displayed. (Contributed by Gregory P. Smith.)

• The glob.glob() function can now return Unicode filenames if a Unicode path was used and Unicode filenames are matched within the directory. (bpo-1001604)

• A new function in the heapq module, merge(iter1, iter2, ...), takes any number of iterables returning data in sorted order, and returns a new generator that returns the contents of all the iterators, also in sorted order. For example:

  >>> list(heapq.merge([1, 3, 5, 9], [2, 8, 16]))
  [1, 2, 3, 5, 8, 9, 16]
  

  Another new function, heappushpop(heap, item), pushes item onto heap, then pops off and returns the smallest item. This is more efficient than making a call to heappush() and then heappop().

  heapq is now implemented to only use less-than comparison, instead of the less-than-or-equal comparison it previously used. This makes heapq’s usage of a type match the list.sort() method. (Contributed by Raymond Hettinger.)

• An optional timeout parameter, specifying a timeout measured in seconds, was added to the httplib.HTTPConnection and HTTPSConnection class constructors. (Added by Facundo Batista.)

• Most of the inspect module’s functions, such as getmoduleinfo() and getargs(), now return named tuples. In addition to behaving like tuples, the elements of the return value can also be accessed as attributes. (Contributed by Raymond Hettinger.)

  Some new functions in the module include isgenerator(), isgeneratorfunction(), and isabstract().

• The itertools module gained several new functions.

  izip_longest(iter1, iter2, ...[, fillvalue]) makes tuples from each of the elements; if some of the iterables are shorter than others, the missing values are set to fillvalue. For example:

  >>> tuple(itertools.izip_longest([1,2,3], [1,2,3,4,5]))
  ((1, 1), (2, 2), (3, 3), (None, 4), (None, 5))
  

  product(iter1, iter2, ..., [repeat=N]) returns the Cartesian product of the supplied iterables, a set of tuples containing every possible combination of the elements returned from each iterable.

  >>> list(itertools.product([1,2,3], [4,5,6]))
  [(1, 4), (1, 5), (1, 6),
   (2, 4), (2, 5), (2, 6),
   (3, 4), (3, 5), (3, 6)]
  

  The optional repeat keyword argument is used for taking the product of an iterable or a set of iterables with themselves, repeated N times. With a single iterable argument, N-tuples are returned:

  >>> list(itertools.product([1,2], repeat=3))
  [(1, 1, 1), (1, 1, 2), (1, 2, 1), (1, 2, 2),
   (2, 1, 1), (2, 1, 2), (2, 2, 1), (2, 2, 2)]
  

  With two iterables, 2N-tuples are returned.

  >>> list(itertools.product([1,2], [3,4], repeat=2))
  [(1, 3, 1, 3), (1, 3, 1, 4), (1, 3, 2, 3), (1, 3, 2, 4),
   (1, 4, 1, 3), (1, 4, 1, 4), (1, 4, 2, 3), (1, 4, 2, 4),
   (2, 3, 1, 3), (2, 3, 1, 4), (2, 3, 2, 3), (2, 3, 2, 4),
   (2, 4, 1, 3), (2, 4, 1, 4), (2, 4, 2, 3), (2, 4, 2, 4)]
  

  combinations(iterable, r) returns sub-sequences of length r from the elements of iterable.

  >>> list(itertools.combinations('123', 2))
  [('1', '2'), ('1', '3'), ('2', '3')]
  >>> list(itertools.combinations('123', 3))
  [('1', '2', '3')]
  >>> list(itertools.combinations('1234', 3))
  [('1', '2', '3'), ('1', '2', '4'),
   ('1', '3', '4'), ('2', '3', '4')]
  

  permutations(iter[, r]) returns all the permutations of length r of the iterable’s elements. If r is not specified, it will default to the number of elements produced by the iterable.

  >>> list(itertools.permutations([1,2,3,4], 2))
  [(1, 2), (1, 3), (1, 4),
   (2, 1), (2, 3), (2, 4),
   (3, 1), (3, 2), (3, 4),
   (4, 1), (4, 2), (4, 3)]
  

  itertools.chain(*iterables) is an existing function in itertools that gained a new constructor in Python 2.6. itertools.chain.from_iterable(iterable) takes a single iterable that should return other iterables. chain() will then return all the elements of the first iterable, then all the elements of the second, and so on.

  >>> list(itertools.chain.from_iterable([[1,2,3], [4,5,6]]))
  [1, 2, 3, 4, 5, 6]
  

  (All contributed by Raymond Hettinger.)

• The logging module’s FileHandler class and its subclasses WatchedFileHandler, RotatingFileHandler, and TimedRotatingFileHandler now have an optional delay parameter to their constructors. If delay is true, opening of the log file is deferred until the first emit() call is made. (Contributed by Vinay Sajip.)

  TimedRotatingFileHandler also has a utc constructor parameter. If the argument is true, UTC time will be used in determining when midnight occurs and in generating filenames; otherwise local time will be used.

• Several new functions were added to the math module:

  • isinf() and isnan() determine whether a given float is a (positive or negative) infinity or a NaN (Not a Number), respectively.

  • copysign() copies the sign bit of an IEEE 754 number, returning the absolute value of x combined with the sign bit of y. For example, math.copysign(1, -0.0) returns -1.0. (Contributed by Christian Heimes.)

  • factorial() computes the factorial of a number. (Contributed by Raymond Hettinger; bpo-2138.)

  • fsum() adds up the stream of numbers from an iterable, and is careful to avoid loss of precision through using partial sums. (Contributed by Jean Brouwers, Raymond Hettinger, and Mark Dickinson; bpo-2819.)

  • acosh(), asinh() and atanh() compute the inverse hyperbolic functions.

  • log1p() returns the natural logarithm of 1+x (base e).

  • trunc() rounds a number toward zero, returning the closest Integral that’s between the function’s argument and zero. Added as part of the backport of PEP 3141’s type hierarchy for numbers.

• The math module has been improved to give more consistent behaviour across platforms, especially with respect to handling of floating-point exceptions and IEEE 754 special values.

  Whenever possible, the module follows the recommendations of the C99 standard about 754’s special values. For example, sqrt(-1.) should now give a ValueError across almost all platforms, while sqrt(float('NaN')) should return a NaN on all IEEE 754 platforms. Where Annex “F” of the C99 standard recommends signaling “divide-by-zero” or “invalid”, Python will raise ValueError. Where Annex “F” of the C99 standard recommends signaling “overflow”, Python will raise OverflowError. (See bpo-711019 and bpo-1640.)

  (Contributed by Christian Heimes and Mark Dickinson.)

• mmap objects now have a rfind() method that searches for a substring beginning at the end of the string and searching backwards. The find() method also gained an end parameter giving an index at which to stop searching. (Contributed by John Lenton.)

• The operator module gained a methodcaller() function that takes a name and an optional set of arguments, returning a callable that will call the named function on any arguments passed to it. For example:

  >>> # Equivalent to lambda s: s.replace('old', 'new')
  >>> replacer = operator.methodcaller('replace', 'old', 'new')
  >>> replacer('old wine in old bottles')
  'new wine in new bottles'
  

  (Contributed by Georg Brandl, after a suggestion by Gregory Petrosyan.)

  The attrgetter() function now accepts dotted names and performs the corresponding attribute lookups:

  >>> inst_name = operator.attrgetter(
  ...        '__class__.__name__')
  >>> inst_name('')
  'str'
  >>> inst_name(help)
  '_Helper'
  

  (Contributed by Georg Brandl, after a suggestion by Barry Warsaw.)

• The os module now wraps several new system calls. fchmod(fd, mode) and fchown(fd, uid, gid) change the mode and ownership of an opened file, and lchmod(path, mode) changes the mode of a symlink. (Contributed by Georg Brandl and Christian Heimes.)

  chflags() and lchflags() are wrappers for the corresponding system calls (where they’re available), changing the flags set on a file. Constants for the flag values are defined in the stat module; some possible values include UF_IMMUTABLE to signal the file may not be changed and UF_APPEND to indicate that data can only be appended to the file. (Contributed by M. Levinson.)

  os.closerange(low, high) efficiently closes all file descriptors from low to high, ignoring any errors and not including high itself. This function is now used by the subprocess module to make starting processes faster. (Contributed by Georg Brandl; bpo-1663329.)

• The os.environ object’s clear() method will now unset the environment variables using os.unsetenv() in addition to clearing the object’s keys. (Contributed by Martin Horcicka; bpo-1181.)

• The os.walk() function now has a followlinks parameter. If set to True, it will follow symlinks pointing to directories and visit the directory’s contents. For backward compatibility, the parameter’s default value is false. Note that the function can fall into an infinite recursion if there’s a symlink that points to a parent directory. (bpo-1273829)

• In the os.path module, the splitext() function has been changed to not split on leading period characters. This produces better results when operating on Unix’s dot-files. For example, os.path.splitext('.ipython') now returns ('.ipython', '') instead of ('', '.ipython'). (bpo-1115886)

  A new function, os.path.relpath(path, start='.'), returns a relative path from the start path, if it’s supplied, or from the current working directory to the destination path. (Contributed by Richard Barran; bpo-1339796.)

  On Windows, os.path.expandvars() will now expand environment variables given in the form «%var%», and «~user» will be expanded into the user’s home directory path. (Contributed by Josiah Carlson; bpo-957650.)

• The Python debugger provided by the pdb module gained a new command: «run» restarts the Python program being debugged and can optionally take new command-line arguments for the program. (Contributed by Rocky Bernstein; bpo-1393667.)

• The pdb.post_mortem() function, used to begin debugging a traceback, will now use the traceback returned by sys.exc_info() if no traceback is supplied. (Contributed by Facundo Batista; bpo-1106316.)

• The pickletools module now has an optimize() function that takes a string containing a pickle and removes some unused opcodes, returning a shorter pickle that contains the same data structure. (Contributed by Raymond Hettinger.)

• A get_data() function was added to the pkgutil module that returns the contents of resource files included with an installed Python package. For example:

  >>> import pkgutil
  >>> print pkgutil.get_data('test', 'exception_hierarchy.txt')
  BaseException
   +-- SystemExit
   +-- KeyboardInterrupt
   +-- GeneratorExit
   +-- Exception
        +-- StopIteration
        +-- StandardError
   ...
  

  (Contributed by Paul Moore; bpo-2439.)

• The pyexpat module’s Parser objects now allow setting their buffer_size attribute to change the size of the buffer used to hold character data. (Contributed by Achim Gaedke; bpo-1137.)

• The Queue module now provides queue variants that retrieve entries in different orders. The PriorityQueue class stores queued items in a heap and retrieves them in priority order, and LifoQueue retrieves the most recently added entries first, meaning that it behaves like a stack. (Contributed by Raymond Hettinger.)

• The random module’s Random objects can now be pickled on a 32-bit system and unpickled on a 64-bit system, and vice versa. Unfortunately, this change also means that Python 2.6’s Random objects can’t be unpickled correctly on earlier versions of Python. (Contributed by Shawn Ligocki; bpo-1727780.)

  The new triangular(low, high, mode) function returns random numbers following a triangular distribution. The returned values are between low and high, not including high itself, and with mode as the most frequently occurring value in the distribution. (Contributed by Wladmir van der Laan and Raymond Hettinger; bpo-1681432.)

• Long regular expression searches carried out by the re module will check for signals being delivered, so time-consuming searches can now be interrupted. (Contributed by Josh Hoyt and Ralf Schmitt; bpo-846388.)

  The regular expression module is implemented by compiling bytecodes for a tiny regex-specific virtual machine. Untrusted code could create malicious strings of bytecode directly and cause crashes, so Python 2.6 includes a verifier for the regex bytecode. (Contributed by Guido van Rossum from work for Google App Engine; bpo-3487.)

• The rlcompleter module’s Completer.complete() method will now ignore exceptions triggered while evaluating a name. (Fixed by Lorenz Quack; bpo-2250.)

• The sched module’s scheduler instances now have a read-only queue attribute that returns the contents of the scheduler’s queue, represented as a list of named tuples with the fields (time, priority, action, argument). (Contributed by Raymond Hettinger; bpo-1861.)

• The select module now has wrapper functions for the Linux epoll() and BSD kqueue() system calls. modify() method was added to the existing poll objects; pollobj.modify(fd, eventmask) takes a file descriptor or file object and an event mask, modifying the recorded event mask for that file. (Contributed by Christian Heimes; bpo-1657.)

• The shutil.copytree() function now has an optional ignore argument that takes a callable object. This callable will receive each directory path and a list of the directory’s contents, and returns a list of names that will be ignored, not copied.

  The shutil module also provides an ignore_patterns() function for use with this new parameter. ignore_patterns() takes an arbitrary number of glob-style patterns and returns a callable that will ignore any files and directories that match any of these patterns. The following example copies a directory tree, but skips both .svn directories and Emacs backup files, which have names ending with “~”:

  shutil.copytree('Doc/library', '/tmp/library',
                  ignore=shutil.ignore_patterns('*~', '.svn'))
  

  (Contributed by Tarek Ziadé; bpo-2663.)

• Integrating signal handling with GUI handling event loops like those used by Tkinter or GTk+ has long been a problem; most software ends up polling, waking up every fraction of a second to check if any GUI events have occurred. The signal module can now make this more efficient. Calling signal.set_wakeup_fd(fd) sets a file descriptor to be used; when a signal is received, a byte is written to that file descriptor. There’s also a C-level function, PySignal_SetWakeupFd(), for setting the descriptor.

  Event loops will use this by opening a pipe to create two descriptors, one for reading and one for writing. The writable descriptor will be passed to set_wakeup_fd(), and the readable descriptor will be added to the list of descriptors monitored by the event loop via select() or poll(). On receiving a signal, a byte will be written and the main event loop will be woken up, avoiding the need to poll.

  (Contributed by Adam Olsen; bpo-1583.)

  The siginterrupt() function is now available from Python code, and allows changing whether signals can interrupt system calls or not. (Contributed by Ralf Schmitt.)

  The setitimer() and getitimer() functions have also been added (where they’re available). setitimer() allows setting interval timers that will cause a signal to be delivered to the process after a specified time, measured in wall-clock time, consumed process time, or combined process+system time. (Contributed by Guilherme Polo; bpo-2240.)

• The smtplib module now supports SMTP over SSL thanks to the addition of the SMTP_SSL class. This class supports an interface identical to the existing SMTP class. (Contributed by Monty Taylor.) Both class constructors also have an optional timeout parameter that specifies a timeout for the initial connection attempt, measured in seconds. (Contributed by Facundo Batista.)

  An implementation of the LMTP protocol (RFC 2033) was also added to the module. LMTP is used in place of SMTP when transferring e-mail between agents that don’t manage a mail queue. (LMTP implemented by Leif Hedstrom; bpo-957003.)

  SMTP.starttls() now complies with RFC 3207 and forgets any knowledge obtained from the server not obtained from the TLS negotiation itself. (Patch contributed by Bill Fenner; bpo-829951.)

• The socket module now supports TIPC (http://tipc.sourceforge.net/), a high-performance non-IP-based protocol designed for use in clustered environments. TIPC addresses are 4- or 5-tuples. (Contributed by Alberto Bertogli; bpo-1646.)

  A new function, create_connection(), takes an address and connects to it using an optional timeout value, returning the connected socket object. This function also looks up the address’s type and connects to it using IPv4 or IPv6 as appropriate. Changing your code to use create_connection() instead of socket(socket.AF_INET, ...) may be all that’s required to make your code work with IPv6.

• The base classes in the SocketServer module now support calling a handle_timeout() method after a span of inactivity specified by the server’s timeout attribute. (Contributed by Michael Pomraning.) The serve_forever() method now takes an optional poll interval measured in seconds, controlling how often the server will check for a shutdown request. (Contributed by Pedro Werneck and Jeffrey Yasskin; bpo-742598, bpo-1193577.)

• The sqlite3 module, maintained by Gerhard Häring, has been updated from version 2.3.2 in Python 2.5 to version 2.4.1.

• The struct module now supports the C99 _Bool type, using the format character '?'. (Contributed by David Remahl.)

• The Popen objects provided by the subprocess module now have terminate(), kill(), and send_signal() methods. On Windows, send_signal() only supports the SIGTERM signal, and all these methods are aliases for the Win32 API function TerminateProcess(). (Contributed by Christian Heimes.)

• A new variable in the sys module, float_info, is an object containing information derived from the float.h file about the platform’s floating-point support. Attributes of this object include mant_dig (number of digits in the mantissa), epsilon (smallest difference between 1.0 and the next largest value representable), and several others. (Contributed by Christian Heimes; bpo-1534.)

  Another new variable, dont_write_bytecode, controls whether Python writes any .pyc or .pyo files on importing a module. If this variable is true, the compiled files are not written. The variable is initially set on start-up by supplying the -B switch to the Python interpreter, or by setting the PYTHONDONTWRITEBYTECODE environment variable before running the interpreter. Python code can subsequently change the value of this variable to control whether bytecode files are written or not. (Contributed by Neal Norwitz and Georg Brandl.)

  Information about the command-line arguments supplied to the Python interpreter is available by reading attributes of a named tuple available as sys.flags. For example, the verbose attribute is true if Python was executed in verbose mode, debug is true in debugging mode, etc. These attributes are all read-only. (Contributed by Christian Heimes.)

  A new function, getsizeof(), takes a Python object and returns the amount of memory used by the object, measured in bytes. Built-in objects return correct results; third-party extensions may not, but can define a __sizeof__() method to return the object’s size. (Contributed by Robert Schuppenies; bpo-2898.)

  It’s now possible to determine the current profiler and tracer functions by calling sys.getprofile() and sys.gettrace(). (Contributed by Georg Brandl; bpo-1648.)

• The tarfile module now supports POSIX.1-2001 (pax) tarfiles in addition to the POSIX.1-1988 (ustar) and GNU tar formats that were already supported. The default format is GNU tar; specify the format parameter to open a file using a different format:

  tar = tarfile.open("output.tar", "w",
                     format=tarfile.PAX_FORMAT)
  

  The new encoding and errors parameters specify an encoding and an error handling scheme for character conversions. 'strict', 'ignore', and 'replace' are the three standard ways Python can handle errors,; 'utf-8' is a special value that replaces bad characters with their UTF-8 representation. (Character conversions occur because the PAX format supports Unicode filenames, defaulting to UTF-8 encoding.)

  The TarFile.add() method now accepts an exclude argument that’s a function that can be used to exclude certain filenames from an archive. The function must take a filename and return true if the file should be excluded or false if it should be archived. The function is applied to both the name initially passed to add() and to the names of files in recursively-added directories.

  (All changes contributed by Lars Gustäbel).

• An optional timeout parameter was added to the telnetlib.Telnet class constructor, specifying a timeout measured in seconds. (Added by Facundo Batista.)

• The tempfile.NamedTemporaryFile class usually deletes the temporary file it created when the file is closed. This behaviour can now be changed by passing delete=False to the constructor. (Contributed by Damien Miller; bpo-1537850.)

  A new class, SpooledTemporaryFile, behaves like a temporary file but stores its data in memory until a maximum size is exceeded. On reaching that limit, the contents will be written to an on-disk temporary file. (Contributed by Dustin J. Mitchell.)

  The NamedTemporaryFile and SpooledTemporaryFile classes both work as context managers, so you can write with tempfile.NamedTemporaryFile() as tmp: .... (Contributed by Alexander Belopolsky; bpo-2021.)

• The test.test_support module gained a number of context managers useful for writing tests. EnvironmentVarGuard() is a context manager that temporarily changes environment variables and automatically restores them to their old values.

  Another context manager, TransientResource, can surround calls to resources that may or may not be available; it will catch and ignore a specified list of exceptions. For example, a network test may ignore certain failures when connecting to an external web site:

  with test_support.TransientResource(IOError,
                                  errno=errno.ETIMEDOUT):
      f = urllib.urlopen('https://sf.net')
      ...
  

  Finally, check_warnings() resets the warning module’s warning filters and returns an object that will record all warning messages triggered (bpo-3781):

  with test_support.check_warnings() as wrec:
      warnings.simplefilter("always")
      # ... code that triggers a warning ...
      assert str(wrec.message) == "function is outdated"
      assert len(wrec.warnings) == 1, "Multiple warnings raised"
  

  (Contributed by Brett Cannon.)

• The textwrap module can now preserve existing whitespace at the beginnings and ends of the newly-created lines by specifying drop_whitespace=False as an argument:

  >>> S = """This  sentence  has a bunch   of
  ...   extra   whitespace."""
  >>> print textwrap.fill(S, width=15)
  This  sentence
  has a bunch
  of    extra
  whitespace.
  >>> print textwrap.fill(S, drop_whitespace=False, width=15)
  This  sentence
    has a bunch
     of    extra
     whitespace.
  >>>
  

  (Contributed by Dwayne Bailey; bpo-1581073.)

• The threading module API is being changed to use properties such as daemon instead of setDaemon() and isDaemon() methods, and some methods have been renamed to use underscores instead of camel-case; for example, the activeCount() method is renamed to active_count(). Both the 2.6 and 3.0 versions of the module support the same properties and renamed methods, but don’t remove the old methods. No date has been set for the deprecation of the old APIs in Python 3.x; the old APIs won’t be removed in any 2.x version. (Carried out by several people, most notably Benjamin Peterson.)

  The threading module’s Thread objects gained an ident property that returns the thread’s identifier, a nonzero integer. (Contributed by Gregory P. Smith; bpo-2871.)

• The timeit module now accepts callables as well as strings for the statement being timed and for the setup code. Two convenience functions were added for creating Timer instances: repeat(stmt, setup, time, repeat, number) and timeit(stmt, setup, time, number) create an instance and call the corresponding method. (Contributed by Erik Demaine; bpo-1533909.)

• The Tkinter module now accepts lists and tuples for options, separating the elements by spaces before passing the resulting value to Tcl/Tk. (Contributed by Guilherme Polo; bpo-2906.)

• The turtle module for turtle graphics was greatly enhanced by Gregor Lingl. New features in the module include:

  • Better animation of turtle movement and rotation.

  • Control over turtle movement using the new delay(), tracer(), and speed() methods.

  • The ability to set new shapes for the turtle, and to define a new coordinate system.

  • Turtles now have an undo() method that can roll back actions.

  • Simple support for reacting to input events such as mouse and keyboard activity, making it possible to write simple games.

  • A turtle.cfg file can be used to customize the starting appearance of the turtle’s screen.

  • The module’s docstrings can be replaced by new docstrings that have been translated into another language.

  (bpo-1513695)

• An optional timeout parameter was added to the urllib.urlopen() function and the urllib.ftpwrapper class constructor, as well as the urllib2.urlopen() function. The parameter specifies a timeout measured in seconds. For example:

  >>> u = urllib2.urlopen("http://slow.example.com",
                          timeout=3)
  Traceback (most recent call last):
    ...
  urllib2.URLError: <urlopen error timed out>
  >>>
  

  (Added by Facundo Batista.)

• The Unicode database provided by the unicodedata module has been updated to version 5.1.0. (Updated by Martin von Löwis; bpo-3811.)

• The warnings module’s formatwarning() and showwarning() gained an optional line argument that can be used to supply the line of source code. (Added as part of bpo-1631171, which re-implemented part of the warnings module in C code.)

  A new function, catch_warnings(), is a context manager intended for testing purposes that lets you temporarily modify the warning filters and then restore their original values (bpo-3781).

• The XML-RPC SimpleXMLRPCServer and DocXMLRPCServer classes can now be prevented from immediately opening and binding to their socket by passing False as the bind_and_activate constructor parameter. This can be used to modify the instance’s allow_reuse_address attribute before calling the server_bind() and server_activate() methods to open the socket and begin listening for connections. (Contributed by Peter Parente; bpo-1599845.)

  SimpleXMLRPCServer also has a _send_traceback_header attribute; if true, the exception and formatted traceback are returned as HTTP headers «X-Exception» and «X-Traceback». This feature is for debugging purposes only and should not be used on production servers because the tracebacks might reveal passwords or other sensitive information. (Contributed by Alan McIntyre as part of his project for Google’s Summer of Code 2007.)

• The xmlrpclib module no longer automatically converts datetime.date and datetime.time to the xmlrpclib.DateTime type; the conversion semantics were not necessarily correct for all applications. Code using xmlrpclib should convert date and time instances. (bpo-1330538) The code can also handle dates before 1900 (contributed by Ralf Schmitt; bpo-2014) and 64-bit integers represented by using <i8> in XML-RPC responses (contributed by Riku Lindblad; bpo-2985).

• The zipfile module’s ZipFile class now has extract() and extractall() methods that will unpack a single file or all the files in the archive to the current directory, or to a specified directory:

  z = zipfile.ZipFile('python-251.zip')
  
  # Unpack a single file, writing it relative
  # to the /tmp directory.
  z.extract('Python/sysmodule.c', '/tmp')
  
  # Unpack all the files in the archive.
  z.extractall()
  

  (Contributed by Alan McIntyre; bpo-467924.)

  The open(), read() and extract() methods can now take either a filename or a ZipInfo object. This is useful when an archive accidentally contains a duplicated filename. (Contributed by Graham Horler; bpo-1775025.)

  Finally, zipfile now supports using Unicode filenames for archived files. (Contributed by Alexey Borzenkov; bpo-1734346.)

The ast module

The ast module provides an Abstract Syntax Tree representation of Python code, and Armin Ronacher contributed a set of helper functions that perform a variety of common tasks. These will be useful for HTML templating packages, code analyzers, and similar tools that process Python code.

The parse() function takes an expression and returns an AST. The dump() function outputs a representation of a tree, suitable for debugging:

import ast

t = ast.parse("""
d = {}
for i in 'abcdefghijklm':
    d[i + i] = ord(i) - ord('a') + 1
print d
""")
print ast.dump(t)

This outputs a deeply nested tree:

Module(body=[
  Assign(targets=[
    Name(id='d', ctx=Store())
   ], value=Dict(keys=[], values=[]))
  For(target=Name(id='i', ctx=Store()),
      iter=Str(s='abcdefghijklm'), body=[
    Assign(targets=[
      Subscript(value=
        Name(id='d', ctx=Load()),
          slice=
          Index(value=
            BinOp(left=Name(id='i', ctx=Load()), op=Add(),
             right=Name(id='i', ctx=Load()))), ctx=Store())
     ], value=
     BinOp(left=
      BinOp(left=
       Call(func=
        Name(id='ord', ctx=Load()), args=[
          Name(id='i', ctx=Load())
         ], keywords=[], starargs=None, kwargs=None),
       op=Sub(), right=Call(func=
        Name(id='ord', ctx=Load()), args=[
          Str(s='a')
         ], keywords=[], starargs=None, kwargs=None)),
       op=Add(), right=Num(n=1)))
    ], orelse=[])
   Print(dest=None, values=[
     Name(id='d', ctx=Load())
   ], nl=True)
 ])

The literal_eval() method takes a string or an AST representing a literal expression, parses and evaluates it, and returns the resulting value. A literal expression is a Python expression containing only strings, numbers, dictionaries, etc. but no statements or function calls. If you need to evaluate an expression but cannot accept the security risk of using an eval() call, literal_eval() will handle it safely:

>>> literal = '("a", "b", {2:4, 3:8, 1:2})'
>>> print ast.literal_eval(literal)
('a', 'b', {1: 2, 2: 4, 3: 8})
>>> print ast.literal_eval('"a" + "b"')
Traceback (most recent call last):
  ...
ValueError: malformed string

The module also includes NodeVisitor and NodeTransformer classes for traversing and modifying an AST, and functions for common transformations such as changing line numbers.

The future_builtins module

Python 3.0 makes many changes to the repertoire of built-in functions, and most of the changes can’t be introduced in the Python 2.x series because they would break compatibility. The future_builtins module provides versions of these built-in functions that can be imported when writing 3.0-compatible code.

The functions in this module currently include:

• ascii(obj): equivalent to repr(). In Python 3.0, repr() will return a Unicode string, while ascii() will return a pure ASCII bytestring.

• filter(predicate, iterable), map(func, iterable1, ...): the 3.0 versions return iterators, unlike the 2.x builtins which return lists.

• hex(value), oct(value): instead of calling the __hex__() or __oct__() methods, these versions will call the __index__() method and convert the result to hexadecimal or octal. oct() will use the new 0o notation for its result.

The json module: JavaScript Object Notation

The new json module supports the encoding and decoding of Python types in JSON (Javascript Object Notation). JSON is a lightweight interchange format often used in web applications. For more information about JSON, see http://www.json.org.

json comes with support for decoding and encoding most built-in Python types. The following example encodes and decodes a dictionary:

>>> import json
>>> data = {"spam": "foo", "parrot": 42}
>>> in_json = json.dumps(data) # Encode the data
>>> in_json
'{"parrot": 42, "spam": "foo"}'
>>> json.loads(in_json) # Decode into a Python object
{"spam": "foo", "parrot": 42}

It’s also possible to write your own decoders and encoders to support more types. Pretty-printing of the JSON strings is also supported.

json (originally called simplejson) was written by Bob Ippolito.

The plistlib module: A Property-List Parser

The .plist format is commonly used on Mac OS X to store basic data types (numbers, strings, lists, and dictionaries) by serializing them into an XML-based format. It resembles the XML-RPC serialization of data types.

Despite being primarily used on Mac OS X, the format has nothing Mac-specific about it and the Python implementation works on any platform that Python supports, so the plistlib module has been promoted to the standard library.

Using the module is simple:

import sys
import plistlib
import datetime

# Create data structure
data_struct = dict(lastAccessed=datetime.datetime.now(),
                   version=1,
                   categories=('Personal','Shared','Private'))

# Create string containing XML.
plist_str = plistlib.writePlistToString(data_struct)
new_struct = plistlib.readPlistFromString(plist_str)
print data_struct
print new_struct

# Write data structure to a file and read it back.
plistlib.writePlist(data_struct, '/tmp/customizations.plist')
new_struct = plistlib.readPlist('/tmp/customizations.plist')

# read/writePlist accepts file-like objects as well as paths.
plistlib.writePlist(data_struct, sys.stdout)

ctypes Enhancements

Thomas Heller continued to maintain and enhance the ctypes module.

ctypes now supports a c_bool datatype that represents the C99 bool type. (Contributed by David Remahl; bpo-1649190.)

The ctypes string, buffer and array types have improved support for extended slicing syntax, where various combinations of (start, stop, step) are supplied. (Implemented by Thomas Wouters.)

All ctypes data types now support from_buffer() and from_buffer_copy() methods that create a ctypes instance based on a provided buffer object. from_buffer_copy() copies the contents of the object, while from_buffer() will share the same memory area.

A new calling convention tells ctypes to clear the errno or Win32 LastError variables at the outset of each wrapped call. (Implemented by Thomas Heller; bpo-1798.)

You can now retrieve the Unix errno variable after a function call. When creating a wrapped function, you can supply use_errno=True as a keyword parameter to the DLL() function and then call the module-level methods set_errno() and get_errno() to set and retrieve the error value.

The Win32 LastError variable is similarly supported by the DLL(), OleDLL(), and WinDLL() functions. You supply use_last_error=True as a keyword parameter and then call the module-level methods set_last_error() and get_last_error().

The byref() function, used to retrieve a pointer to a ctypes instance, now has an optional offset parameter that is a byte count that will be added to the returned pointer.

Improved SSL Support

Bill Janssen made extensive improvements to Python 2.6’s support for the Secure Sockets Layer by adding a new module, ssl, that’s built atop the OpenSSL library. This new module provides more control over the protocol negotiated, the X.509 certificates used, and has better support for writing SSL servers (as opposed to clients) in Python. The existing SSL support in the socket module hasn’t been removed and continues to work, though it will be removed in Python 3.0.

To use the new module, you must first create a TCP connection in the usual way and then pass it to the ssl.wrap_socket() function. It’s possible to specify whether a certificate is required, and to obtain certificate info by calling the getpeercert() method.

Ver también

The documentation for the ssl module.

Deprecations and Removals

• String exceptions have been removed. Attempting to use them raises a TypeError.

• Changes to the Exception interface as dictated by PEP 352 continue to be made. For 2.6, the message attribute is being deprecated in favor of the args attribute.

• (3.0-warning mode) Python 3.0 will feature a reorganized standard library that will drop many outdated modules and rename others. Python 2.6 running in 3.0-warning mode will warn about these modules when they are imported.

  The list of deprecated modules is: audiodev, bgenlocations, buildtools, bundlebuilder, Canvas, compiler, dircache, dl, fpformat, gensuitemodule, ihooks, imageop, imgfile, linuxaudiodev, mhlib, mimetools, multifile, new, pure, statvfs, sunaudiodev, test.testall, and toaiff.

• The gopherlib module has been removed.

• The MimeWriter module and mimify module have been deprecated; use the email package instead.

• The md5 module has been deprecated; use the hashlib module instead.

• The posixfile module has been deprecated; fcntl.lockf() provides better locking.

• The popen2 module has been deprecated; use the subprocess module.

• The rgbimg module has been removed.

• The sets module has been deprecated; it’s better to use the built-in set and frozenset types.

• The sha module has been deprecated; use the hashlib module instead.

Build and C API Changes

Changes to Python’s build process and to the C API include:

• Python now must be compiled with C89 compilers (after 19 years!). This means that the Python source tree has dropped its own implementations of memmove() and strerror(), which are in the C89 standard library.

• Python 2.6 can be built with Microsoft Visual Studio 2008 (version 9.0), and this is the new default compiler. See the PCbuild directory for the build files. (Implemented by Christian Heimes.)

• On Mac OS X, Python 2.6 can be compiled as a 4-way universal build. The configure script can take a --with-universal-archs=[32-bit|64-bit|all] switch, controlling whether the binaries are built for 32-bit architectures (x86, PowerPC), 64-bit (x86-64 and PPC-64), or both. (Contributed by Ronald Oussoren.)

• The BerkeleyDB module now has a C API object, available as bsddb.db.api. This object can be used by other C extensions that wish to use the bsddb module for their own purposes. (Contributed by Duncan Grisby.)

• The new buffer interface, previously described in the PEP 3118 section, adds PyObject_GetBuffer() and PyBuffer_Release(), as well as a few other functions.

• Python’s use of the C stdio library is now thread-safe, or at least as thread-safe as the underlying library is. A long-standing potential bug occurred if one thread closed a file object while another thread was reading from or writing to the object. In 2.6 file objects have a reference count, manipulated by the PyFile_IncUseCount() and PyFile_DecUseCount() functions. File objects can’t be closed unless the reference count is zero. PyFile_IncUseCount() should be called while the GIL is still held, before carrying out an I/O operation using the FILE * pointer, and PyFile_DecUseCount() should be called immediately after the GIL is re-acquired. (Contributed by Antoine Pitrou and Gregory P. Smith.)

• Importing modules simultaneously in two different threads no longer deadlocks; it will now raise an ImportError. A new API function, PyImport_ImportModuleNoBlock(), will look for a module in sys.modules first, then try to import it after acquiring an import lock. If the import lock is held by another thread, an ImportError is raised. (Contributed by Christian Heimes.)

• Several functions return information about the platform’s floating-point support. PyFloat_GetMax() returns the maximum representable floating point value, and PyFloat_GetMin() returns the minimum positive value. PyFloat_GetInfo() returns an object containing more information from the float.h file, such as "mant_dig" (number of digits in the mantissa), "epsilon" (smallest difference between 1.0 and the next largest value representable), and several others. (Contributed by Christian Heimes; bpo-1534.)

• C functions and methods that use PyComplex_AsCComplex() will now accept arguments that have a __complex__() method. In particular, the functions in the cmath module will now accept objects with this method. This is a backport of a Python 3.0 change. (Contributed by Mark Dickinson; bpo-1675423.)

• Python’s C API now includes two functions for case-insensitive string comparisons, PyOS_stricmp(char*, char*) and PyOS_strnicmp(char*, char*, Py_ssize_t). (Contributed by Christian Heimes; bpo-1635.)

• Many C extensions define their own little macro for adding integers and strings to the module’s dictionary in the init* function. Python 2.6 finally defines standard macros for adding values to a module, PyModule_AddStringMacro and PyModule_AddIntMacro(). (Contributed by Christian Heimes.)

• Some macros were renamed in both 3.0 and 2.6 to make it clearer that they are macros, not functions. Py_Size() became Py_SIZE(), Py_Type() became Py_TYPE(), and Py_Refcnt() became Py_REFCNT(). The mixed-case macros are still available in Python 2.6 for backward compatibility. (bpo-1629)

• Distutils now places C extensions it builds in a different directory when running on a debug version of Python. (Contributed by Collin Winter; bpo-1530959.)

• Several basic data types, such as integers and strings, maintain internal free lists of objects that can be re-used. The data structures for these free lists now follow a naming convention: the variable is always named free_list, the counter is always named numfree, and a macro Py<typename>_MAXFREELIST is always defined.

• A new Makefile target, «make patchcheck», prepares the Python source tree for making a patch: it fixes trailing whitespace in all modified .py files, checks whether the documentation has been changed, and reports whether the Misc/ACKS and Misc/NEWS files have been updated. (Contributed by Brett Cannon.)

  Another new target, «make profile-opt», compiles a Python binary using GCC’s profile-guided optimization. It compiles Python with profiling enabled, runs the test suite to obtain a set of profiling results, and then compiles using these results for optimization. (Contributed by Gregory P. Smith.)

Port-Specific Changes: Windows

• The support for Windows 95, 98, ME and NT4 has been dropped. Python 2.6 requires at least Windows 2000 SP4.

• The new default compiler on Windows is Visual Studio 2008 (version 9.0). The build directories for Visual Studio 2003 (version 7.1) and 2005 (version 8.0) were moved into the PC/ directory. The new PCbuild directory supports cross compilation for X64, debug builds and Profile Guided Optimization (PGO). PGO builds are roughly 10% faster than normal builds. (Contributed by Christian Heimes with help from Amaury Forgeot d’Arc and Martin von Löwis.)

• The msvcrt module now supports both the normal and wide char variants of the console I/O API. The getwch() function reads a keypress and returns a Unicode value, as does the getwche() function. The putwch() function takes a Unicode character and writes it to the console. (Contributed by Christian Heimes.)

• os.path.expandvars() will now expand environment variables in the form «%var%», and «~user» will be expanded into the user’s home directory path. (Contributed by Josiah Carlson; bpo-957650.)

• The socket module’s socket objects now have an ioctl() method that provides a limited interface to the WSAIoctl() system interface.

• The _winreg module now has a function, ExpandEnvironmentStrings(), that expands environment variable references such as %NAME% in an input string. The handle objects provided by this module now support the context protocol, so they can be used in with statements. (Contributed by Christian Heimes.)

  _winreg also has better support for x64 systems, exposing the DisableReflectionKey(), EnableReflectionKey(), and QueryReflectionKey() functions, which enable and disable registry reflection for 32-bit processes running on 64-bit systems. (bpo-1753245)

• The msilib module’s Record object gained GetInteger() and GetString() methods that return field values as an integer or a string. (Contributed by Floris Bruynooghe; bpo-2125.)

Port-Specific Changes: Mac OS X

• When compiling a framework build of Python, you can now specify the framework name to be used by providing the --with-framework-name= option to the configure script.

• The macfs module has been removed. This in turn required the macostools.touched() function to be removed because it depended on the macfs module. (bpo-1490190)

• Many other Mac OS modules have been deprecated and will be removed in Python 3.0: _builtinSuites, aepack, aetools, aetypes, applesingle, appletrawmain, appletrunner, argvemulator, Audio_mac, autoGIL, Carbon, cfmfile, CodeWarrior, ColorPicker, EasyDialogs, Explorer, Finder, FrameWork, findertools, ic, icglue, icopen, macerrors, MacOS, macfs, macostools, macresource, MiniAEFrame, Nav, Netscape, OSATerminology, pimp, PixMapWrapper, StdSuites, SystemEvents, Terminal, and terminalcommand.

Port-Specific Changes: IRIX

A number of old IRIX-specific modules were deprecated and will be removed in Python 3.0: al and AL, cd, cddb, cdplayer, CL and cl, DEVICE, ERRNO, FILE, FL and fl, flp, fm, GET, GLWS, GL and gl, IN, IOCTL, jpeg, panelparser, readcd, SV and sv, torgb, videoreader, and WAIT.

Porting to Python 2.6

This section lists previously described changes and other bugfixes that may require changes to your code:

• Classes that aren’t supposed to be hashable should set __hash__ = None in their definitions to indicate the fact.

• String exceptions have been removed. Attempting to use them raises a TypeError.

• The __init__() method of collections.deque now clears any existing contents of the deque before adding elements from the iterable. This change makes the behavior match list.__init__().

• object.__init__() previously accepted arbitrary arguments and keyword arguments, ignoring them. In Python 2.6, this is no longer allowed and will result in a TypeError. This will affect __init__() methods that end up calling the corresponding method on object (perhaps through using super()). See bpo-1683368 for discussion.

• The Decimal constructor now accepts leading and trailing whitespace when passed a string. Previously it would raise an InvalidOperation exception. On the other hand, the create_decimal() method of Context objects now explicitly disallows extra whitespace, raising a ConversionSyntax exception.

• Due to an implementation accident, if you passed a file path to the built-in __import__() function, it would actually import the specified file. This was never intended to work, however, and the implementation now explicitly checks for this case and raises an ImportError.

• C API: the PyImport_Import() and PyImport_ImportModule() functions now default to absolute imports, not relative imports. This will affect C extensions that import other modules.

• C API: extension data types that shouldn’t be hashable should define their tp_hash slot to PyObject_HashNotImplemented().

• The socket module exception socket.error now inherits from IOError. Previously it wasn’t a subclass of StandardError but now it is, through IOError. (Implemented by Gregory P. Smith; bpo-1706815.)

• The xmlrpclib module no longer automatically converts datetime.date and datetime.time to the xmlrpclib.DateTime type; the conversion semantics were not necessarily correct for all applications. Code using xmlrpclib should convert date and time instances. (bpo-1330538)

• (3.0-warning mode) The Exception class now warns when accessed using slicing or index access; having Exception behave like a tuple is being phased out.

• (3.0-warning mode) inequality comparisons between two dictionaries or two objects that don’t implement comparison methods are reported as warnings. dict1 == dict2 still works, but dict1 < dict2 is being phased out.

  Comparisons between cells, which are an implementation detail of Python’s scoping rules, also cause warnings because such comparisons are forbidden entirely in 3.0.

Agradecimientos

The author would like to thank the following people for offering suggestions, corrections and assistance with various drafts of this article: Georg Brandl, Steve Brown, Nick Coghlan, Ralph Corderoy, Jim Jewett, Kent Johnson, Chris Lambacher, Martin Michlmayr, Antoine Pitrou, Brian Warner.