Qué hay de nuevo en Python 2.7
******************************

Autor:
   A.M. Kuchling (amk en amk.ca)

Este articulo explica las nuevas características en Python 2.7.
Python 2.7 fue publicado el 3 de Julio de 2010.

El manejo numérico ha sido mejorado en muchas formas, tanto para los
números de punto flotante como para la clase "Decimal". Hay algunas
adiciones útiles a la biblioteca estándar, como una gran mejora al
modulo "unittest", el modulo "argparse" para analizar las opciones de
la línea de comandos, adecuando las clases "OrderedDict" y "Counter"
en el modulo "collections", y muchas otras mejoras.

Python 2.7 esta previsto que sea el ultimo lanzamiento de la serie
2.x, así que hemos trabajado para que sea una versión a largo plazo.
Para ayudar con la migración a Python 3, se han incluido varias
características nuevas de la serie Python 3.x en 2.7.

Este articulo no intenta proporcionar una especificación completa de
las nuevas características, sino que proporciona una visión general
conveniente.  Para obtener mas información, debería consultar la
documentación de Python 2.7 en https://docs.python.org. Si desea
comprender la justificación para el diseño y la implementación,
consultar el PEP para una nueva característica particular o el error
en el que se discutió un cambio en https://bugs.python.org.  Siempre
que sea posible, "Que hay de nuevo en Python" enlaza el elemento
error/parche para cada cambio.


El futuro de Python 2.x
=======================

Python 2.7 is the last major release in the 2.x series, as the Python
maintainers have shifted the focus of their new feature development
efforts to the Python 3.x series. This means that while Python 2
continues to receive bug fixes, and to be updated to build correctly
on new hardware and versions of supported operated systems, there will
be no new full feature releases for the language or standard library.

Sin embargo, si bien existe un gran subconjunto común entre Python 2.7
y Python 3, y muchos de los cambios involucrados en la migración a ese
subconjunto común, o directamente a Python 3, se pueden automatizar de
manera segura, algunos otros cambios (en particular los asociados con
el manejo de Unicode) puede requerir una consideración cuidadosa, y
preferiblemente conjuntos de pruebas de regresión automatizada
robustos, para migrar de manera efectiva.

Esto significa que Python 2.7 permanecerá en su lugar durante un largo
tiempo, proporcionando una plataforma base estable y compatible para
sistemas de producción que aún no se han adaptado a Python 3. La
expectativa completa del ciclo de vida de la versión de Python 2.7
esta detallada en **PEP 373**.

Algunas consecuencias clave de la importancia a largo plazo de 2.7
son:

* Como se señalo anteriormente, la versión 2.7 tiene un período de
  mantenimiento mucho más largo en comparación con las versiones
  anteriores 2.x. Actualmente se espera que Python 2.7 siga siendo
  compatible con el equipo de desarrollo central (recibiendo
  actualizaciones de seguridad y otras correcciones de errores) al
  menos hasta 2020 (10 años después de su lanzamiento inicial,
  comparado con el típico periodo de mantenimiento de 18--24 meses).

* A medida que la biblioteca estándar de Python envejece, hacer un uso
  eficaz del índice de paquetes de Python (ya sea directamente o a
  través de un distribuidor) se vuelve mas importante para los
  usuarios de Python 2. Además de una amplia variedad de paquetes de
  terceros para diversas tareas, los paquetes disponibles incluyen
  *backports* de nuevos módulos y características de la biblioteca
  estándar de Python 3 que son compatibles con Python 2, así como
  varias herramientas y librerías que pueden hacer mas fácil la
  migración a Python 3. La Guía de usuario de paquetes de Python
  proporciona un información sobre como descargar e instalar software
  desde el índice de paquetes de Python.

* Aunque el enfoque preferido para mejorar Python 2 es ahora la
  publicación de nuevos paquetes en el índice de paquetes de Python,
  este enfoque no necesariamente funciona en todos los casos,
  especialmente aquellos relacionados a la seguridad de la red. En
  casos excepcionales que no puedan ser manejados adecuadamente
  mediante la publicación de nuevos paquetes o actualizaciones en
  PyPI, el proceso de propuestas de mejoras de Python podría ser usado
  para crear el caso para añadir nuevas funciones a la librería
  estándar de Python 2. Dichas adiciones, y las versiones de
  mantenimiento donde fueron agregadas, se observaran en la sección
  New Features Added to Python 2.7 Maintenance Releases mas abajo.

Para los proyectos que deseen migrar de Python 2 a Python 3, o para
los desarrolladores de *frameworks* y libreras que deseen dar soporte
a usuarios en Python 2 y Python 3, hay una variedad de herramientas y
guías disponibles para ayudarles a decidir sobre un enfoque adecuado y
manejar algunos de los detalles técnicos involucrados. El punto de
partida recomendado es la guía HOWTO Portando código de Python 2 a
Python 3.


Cambios en el manejo de las advertencias de desuso
==================================================

Para Python 2.7, se tomo la decisión de silenciar las advertencias
solo de interés para los desarrolladores de manera predeterminada.
"DeprecationWarning`y sus descendientes son ahora ignoradas a menos
que se solicite lo contrario, impidiendo a los usuarios estar viendo
las advertencias disparadas por una aplicación. Este cambio se realizo
en la rama que se convirtió en Python 3.2. (Discutido en stdlib-sig y
llevado a cabo en :issue:`7319").

En versiones previas, los mensajes "DeprecationWarning" estaban
habilitados de manera predeterminada, proporcionando a los
desarrolladores de Python una indicación clara de donde podría
romperse su código en una futura versión principal de Python.

Sin embargo, cada vez hay más usuarios de aplicaciones basadas en
Python que no están directamente involucrados en el desarrollo de esas
aplicaciones.  Los mensajes "DeprecationWarning" son irrelevantes para
estos usuarios, hacen que se preocupen por una aplicación que
realmente está funcionando correctamente y sobrecargando a los
desarrolladores de aplicaciones con responder a estas preocupaciones.

Puede volver a habilitar la visualización de los mensajes
"DeprecationWarning" ejecutando Python con el modificador "-Wdefault"
(forma corta: "-Wd"), o estableciendo la variable de entorno
"PYTHONWARNINGS" en ""default"" (o ""d"") antes de ejecutar Python.
El código Python también puede volver a habilitarlos llamando a
''warnings.simplefilter('default')``.

El módulo "unittest" también vuelve a instalar automáticamente las
advertencias de desuso al ejecutar pruebas.


Características de Python 3.1
=============================

Al igual que Python 2.6 incorporó características de Python 3.0, la
versión 2.7 incorpora algunas de las nuevas características de Python
3.1.  La serie 2.x continúa proporcionando herramientas para migrar a
la serie 3.x.

Una lista parcial de características 3.1 que se retro importaron a
2.7:

* La sintaxis para establecer literales ("{1,2,3}" es un conjunto
  mutable).

* Diccionario y establecimiento de compresiones ("{i: i*2 for i in
  range(3)}").

* Múltiples administradores de contexto en una sola declaración
  "with".

* Una nueva versión para la librería "io", rescrito en C para un mejor
  desempeño.

* El tipo de ordenador de diccionario descrito en PEP 372: Adición de
  un diccionario ordenado a las colecciones.

* El nuevo especificador de formato "","" descrito en PEP 378:
  Especificador de formato para separador de miles.

* El objeto "memoryview".

* Un pequeño subconjunto del modulo "importlib", descrito abajo.

* La "repr()" de un flotante "x" es más corto en muchos casos: ahora
  se basa en la cadena decimal más corta que se garantiza que redondea
  a "x". Como en versiones anteriores de Python, se garantiza que
  "float(repr(x))" devuelva "x".

* Las conversiones flotante a cadena y cadena a flotante se redondean
  correctamente. La función "round()" también ahora se redondea
  correctamente.

* El tipo "PyCapsule", utilizado para proporcionar una API de C para
  módulos de extensión.

* La función de API de C "PyLong_AsLongAndOverflow()".

Otras nuevas advertencias en modo Python3 incluyen:

* "operator.isCallable()" y "operator.sequenceIncludes()", que no se
  admiten en 3.x, ahora activan advertencias.

* El modificador "-3" ahora habilita automáticamente el modificador
  "-Qwarn" que provoca advertencias sobre el uso de la división
  clásica con enteros y enteros largos.


PEP 372: Adición de un diccionario ordenado a las colecciones
=============================================================

Los diccionarios de python normales recorren en iteración los pares
clave/valor en un orden arbitrario. A lo largo de los años, varios
autores han escrito implementaciones alternativas que recuerdan el
orden en que se insertaron originalmente las claves.  Basado en las
experiencias de esas implementaciones, 2.7 presenta una nueva clase
"OrderedDict" en el módulo "collections".

La API "OrderedDict" proporciona la misma interfaz que los
diccionarios normales, pero recorre en iteración las claves y los
valores en un orden garantizado en función de cuándo se insertó una
clave por primera vez:

   >>> from collections import OrderedDict
   >>> d = OrderedDict([('first', 1),
   ...                  ('second', 2),
   ...                  ('third', 3)])
   >>> d.items()
   [('first', 1), ('second', 2), ('third', 3)]

Si una nueva entrada sobrescribe una entrada existente, la posición de
inserción original no se modifica:

   >>> d['second'] = 4
   >>> d.items()
   [('first', 1), ('second', 4), ('third', 3)]

Borrando una entrada y luego reinsertándola, moverá la misma al final:

   >>> del d['second']
   >>> d['second'] = 5
   >>> d.items()
   [('first', 1), ('third', 3), ('second', 5)]

El método "popitem()" tiene un argumento *last* opcional que por
defecto esta en "True" (verdadero). Si *last* es verdadero, la ultima
llave ingresada es regresada y eliminada; si es falso, se selecciona
la llave más antigua:

   >>> od = OrderedDict([(x,0) for x in range(20)])
   >>> od.popitem()
   (19, 0)
   >>> od.popitem()
   (18, 0)
   >>> od.popitem(last=False)
   (0, 0)
   >>> od.popitem(last=False)
   (1, 0)

La comparación de dos diccionarios ordenados comprueba tanto las
claves como los valores, y requiere que el orden de inserción sea el
mismo:

   >>> od1 = OrderedDict([('first', 1),
   ...                    ('second', 2),
   ...                    ('third', 3)])
   >>> od2 = OrderedDict([('third', 3),
   ...                    ('first', 1),
   ...                    ('second', 2)])
   >>> od1 == od2
   False
   >>> # Move 'third' key to the end
   >>> del od2['third']; od2['third'] = 3
   >>> od1 == od2
   True

Comparando a un "OrderedDict" con un diccionario normal ignora el
orden de inserción y simplemente compara las llaves y los valores.

¿Cómo funciona "OrderedDict"?  Mantiene una lista doblemente vinculada
de claves, añadiendo nuevas claves a la lista a medida que se
insertan. Un diccionario secundario asigna las claves a su nodo de
lista correspondiente, por lo que la eliminación no tiene que recorrer
toda la lista vinculada y, por lo tanto, sigue siendo O(1).

La biblioteca estándar ahora admite el uso de diccionarios ordenados
en varios módulos.

* El módulo "ConfigParser" los utiliza de forma predeterminada, lo que
  significa que los archivos de configuración ahora se pueden leer,
  modificar y volver a escribir en su orden original.

* El método "_asdict()" para "collections.namedtuple()" ahora devuelve
  un diccionario ordenado con los valores que aparecen en el mismo
  orden que los índices de tupla subyacentes.

* El módulo "json" de la clase constructor "JSONDecoder" se amplió con
  un parámetro *object_pairs_hook* para permitir que el descodificador
  compilara instancias "OrderedDict". También se agregó compatibilidad
  con herramientas de terceros como PyYAML.

Ver también:

  **PEP 372** - Adición de un diccionario ordenado a las *collections*
     PEP escrito por Armin Ronacher y Raymond Hettinger; implementado
     por Raymond Hettinger.


PEP 378: Especificador de formato para separador de miles
=========================================================

Para que la salida del programa sea más legible, puede ser útil
agregar separadores a números grandes, representándolos como
18,446,744,073,709,551,616 en lugar de 18446744073709551616.

La solución totalmente general para hacer esto es el módulo "locale",
que puede utilizar diferentes separadores ("," en América del Norte,
"." en Europa) y diferentes tamaños de agrupación, pero "locale" es
complicado de usar y no es adecuado para aplicaciones multiproceso
donde diferentes hilos están produciendo resultados para diferentes
configuraciones regionales.

Por lo tanto, se ha añadido un mecanismo simple de agrupación de comas
al mini-lenguaje utilizado por el método "str.format()".  Al dar
formato a un número de punto flotante, simplemente incluya una coma
entre el ancho y la precisión:

   >>> '{:20,.2f}'.format(18446744073709551616.0)
   '18,446,744,073,709,551,616.00'

Al dar formato a un entero, incluya la coma después del ancho:

>>> '{:20,d}'.format(18446744073709551616)
'18,446,744,073,709,551,616'

Este mecanismo no es adaptable en absoluto; Las comas siempre se
utilizan como separador y la agrupación siempre está en grupos de tres
dígitos.  El mecanismo de formato de coma no es tan general como el
módulo "locale", pero es más fácil de usar.

Ver también:

  **PEP 378** - Especificador de formato para separador de miles
     PEP escrito por Raymond Hettinger; implementado por Eric Smith.


PEP 389: El módulo argparse para el análisis de líneas de comando
=================================================================

El módulo "argparse" para analizar argumentos de línea de comandos se
agregó como un reemplazo más potente para el módulo "optparse".

Esto significa que Python ahora admite tres módulos diferentes para
analizar argumentos de línea de comandos: "getopt", "optparse", y
"argparse".  El módulo "getopt" se parece mucho a la función
"getopt()" de la biblioteca C, por lo que sigue siendo útil si estás
escribiendo un prototipo de Python que finalmente se reescribe en C.
"optparse" se vuelve redundante, pero no hay planes para eliminarlo
porque hay muchos scripts que todavía lo usan, y no hay una manera
automatizada de actualizar estos scripts.  (Hacer que la API
"argparse" sea coherente con la interfaz de "optparse" fue discutido
pero rechazado como demasiado desordenado y difícil.)

En resumen, si estás escribiendo un nuevo script y no necesitas
preocuparte por la compatibilidad con versiones anteriores de Python,
usa "argparse" en lugar de "optparse".

Este es un ejemplo:

   import argparse

   parser = argparse.ArgumentParser(description='Command-line example.')

   # Add optional switches
   parser.add_argument('-v', action='store_true', dest='is_verbose',
                       help='produce verbose output')
   parser.add_argument('-o', action='store', dest='output',
                       metavar='FILE',
                       help='direct output to FILE instead of stdout')
   parser.add_argument('-C', action='store', type=int, dest='context',
                       metavar='NUM', default=0,
                       help='display NUM lines of added context')

   # Allow any number of additional arguments.
   parser.add_argument(nargs='*', action='store', dest='inputs',
                       help='input filenames (default is stdin)')

   args = parser.parse_args()
   print args.__dict__

A menos que lo reemplace, los modificadores "-h" y "--help" se agregan
automáticamente, y producen una salida con formato ordenado:

   -> ./python.exe argparse-example.py --help
   usage: argparse-example.py [-h] [-v] [-o FILE] [-C NUM] [inputs [inputs ...]]

   Command-line example.

   positional arguments:
     inputs      input filenames (default is stdin)

   optional arguments:
     -h, --help  show this help message and exit
     -v          produce verbose output
     -o FILE     direct output to FILE instead of stdout
     -C NUM      display NUM lines of added context

Al igual que con "optparse", los modificadores y argumentos de línea
de comandos se devuelven como un objeto con atributos denominados por
los parámetros *dest*:

   -> ./python.exe argparse-example.py -v
   {'output': None,
    'is_verbose': True,
    'context': 0,
    'inputs': []}

   -> ./python.exe argparse-example.py -v -o /tmp/output -C 4 file1 file2
   {'output': '/tmp/output',
    'is_verbose': True,
    'context': 4,
    'inputs': ['file1', 'file2']}

"argparse" tiene una validación mucho más fantasiosa que "optparse";
puede especificar un número exacto de argumentos como un entero, 0 o
más argumentos pasando "'*'", 1 o más pasando "'+'", o un argumento
opcional con "'?'".  Un analizador de nivel superior puede contener
sub analizadores para definir subcomandos que tienen diferentes
conjuntos de modificadores, como en "svn commit", "svn checkout", etc.
Puede especificar el tipo de un argumento como "FileType", que abrirá
automáticamente los archivos y entiende que "'-'" significa entrada o
salida estándar.

Ver también:

  "argparse" documentación
     La página de documentación del módulo argparse.

  Actualizar el código de optparse
     Parte de la documentación de Python, que describe cómo convertir
     código que usa "optparse".

  **PEP 389** - argparse - Nuevo módulo de análisis de línea de
  comandos
     PEP escrito e implementado por Steven Bethard.


PEP 391: Configuración basada en diccionarios para el registro
==============================================================

El módulo "logging" es muy flexible; las aplicaciones pueden definir
un árbol de subsistemas de registro, y cada registrador de este árbol
puede filtrar ciertos mensajes, formatearlos de forma diferente y
dirigir mensajes a un número variable de controladores.

Toda esta flexibilidad puede requerir mucha configuración.  Puede
escribir instrucciones Python para crear objetos y establecer sus
propiedades, pero una configuración compleja requiere código detallado
pero aburrido. "logging" también es compatible con una función
"fileConfig()" que analiza un archivo, pero el formato de archivo no
admite la configuración de filtros, y es más complicado generar
mediante programación.

Python 2.7 agrega una función "dictConfig()" que utiliza un
diccionario para configurar el registro.  Hay muchas maneras de
producir un diccionario de diferentes fuentes: construir uno con
código; analizar un archivo que contenga JSON; o utilice una
biblioteca de análisis YAML si hay una instalada.  Para obtener más
información, consulte Funciones de configuración.

En el ejemplo siguiente se configuran dos registradores, el
registrador raíz y un registrador denominado "network".  Los mensajes
enviados al registrador raíz se enviarán al registro del sistema
mediante el protocolo syslog, y los mensajes al registrador de la
"network" se escribirán en un archivo "network.log" que se rotará una
vez que el registro alcance 1MB.

   import logging
   import logging.config

   configdict = {
    'version': 1,    # Configuration schema in use; must be 1 for now
    'formatters': {
        'standard': {
            'format': ('%(asctime)s %(name)-15s '
                       '%(levelname)-8s %(message)s')}},

    'handlers': {'netlog': {'backupCount': 10,
                        'class': 'logging.handlers.RotatingFileHandler',
                        'filename': '/logs/network.log',
                        'formatter': 'standard',
                        'level': 'INFO',
                        'maxBytes': 1000000},
                 'syslog': {'class': 'logging.handlers.SysLogHandler',
                            'formatter': 'standard',
                            'level': 'ERROR'}},

    # Specify all the subordinate loggers
    'loggers': {
                'network': {
                            'handlers': ['netlog']
                }
    },
    # Specify properties of the root logger
    'root': {
             'handlers': ['syslog']
    },
   }

   # Set up configuration
   logging.config.dictConfig(configdict)

   # As an example, log two error messages
   logger = logging.getLogger('/')
   logger.error('Database not found')

   netlogger = logging.getLogger('network')
   netlogger.error('Connection failed')

Tres mejoras más pequeñas en el módulo "logging", todas implementadas
por Vinay Sajip, son:

* La clase "SysLogHandler" ahora admite syslogging a través de TCP.
  El constructor tiene un parámetro *socktype* que proporciona el tipo
  de socket que se va a usar, ya sea "socket.SOCK_DGRAM" para UDP o
  "socket.SOCK_STREAM" para TCP.  El protocolo predeterminado sigue
  siendo UDP.

* Las instancias "Logger" ganaron un método "getChild()" que recupera
  un registrador descendiente mediante una ruta de acceso relativa.
  Por ejemplo, una vez que se recupera un registrador haciendo "log =
  getLogger('app')", llamando "log.getChild('network.listen')" es
  equivalente a "getLogger('app.network.listen')".

* La clase "LoggerAdapter" gano un método "isEnabledFor()" que toma un
  *nivel* y devuelve si el registrador subyacente procesará un mensaje
  de ese nivel de importancia.

Ver también:

  **PEP 391** - Configuración basada en diccionarios para el registro
     PEP escrito e implementado por Vinay Sajip.


PEP 3106: Vistas de diccionario
===============================

Los métodos de diccionarios "keys()", "values()" y "items()" son
diferentes en Python 3.x. Ellos regresan un objeto llamado *view* en
lugar de una lista completamente materializada.

No es posible cambiar el retorno de valores de los métodos "keys()",
"values()" y "items()" en Python 2.7 porque se rompería demasiado
código. En su lugar en las versiones 3.x se fueron agregando bajo los
nuevos nombres  "viewkeys()", "viewvalues()" y "viewitems()".

   >>> d = dict((i*10, chr(65+i)) for i in range(26))
   >>> d
   {0: 'A', 130: 'N', 10: 'B', 140: 'O', 20: ..., 250: 'Z'}
   >>> d.viewkeys()
   dict_keys([0, 130, 10, 140, 20, 150, 30, ..., 250])

Las vistas se pueden iterar, pero las vistas de elementos y claves
también se comportan como conjuntos. El operador "&" realiza la
intersección y "|" realiza una unión:

   >>> d1 = dict((i*10, chr(65+i)) for i in range(26))
   >>> d2 = dict((i**.5, i) for i in range(1000))
   >>> d1.viewkeys() & d2.viewkeys()
   set([0.0, 10.0, 20.0, 30.0])
   >>> d1.viewkeys() | range(0, 30)
   set([0, 1, 130, 3, 4, 5, 6, ..., 120, 250])

La vista realiza un seguimiento del diccionario y su contenido cambia
a medida que se modifica el diccionario:

   >>> vk = d.viewkeys()
   >>> vk
   dict_keys([0, 130, 10, ..., 250])
   >>> d[260] = '&'
   >>> vk
   dict_keys([0, 130, 260, 10, ..., 250])

Sin embargo, tenga en cuenta que no puede agregar o eliminar claves
mientras está iterando sobre la vista:

   >>> for k in vk:
   ...     d[k*2] = k
   ...
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   RuntimeError: dictionary changed size during iteration

Puede usar los métodos de vista en el código en Python 2.x, y el
convertidor 2to3 los cambiará a los métodos estándar "keys()",
"values()" y "items()".

Ver también:

  **PEP 3106** - Renovación dict.keys(), .values() y .items()
     PEP escrito por Guido van Rossum. Portado a 2.7 por Alexandre
     Vassalotti; bpo-1967.


PEP 3137: El objeto memoryview
==============================

El objeto "memoryview" proporciona una vista del contenido de la
memoria de otro objeto que coincide con la interfaz de tipo  "bytes".

   >>> import string
   >>> m = memoryview(string.letters)
   >>> m
   <memory at 0x37f850>
   >>> len(m)           # Returns length of underlying object
   52
   >>> m[0], m[25], m[26]   # Indexing returns one byte
   ('a', 'z', 'A')
   >>> m2 = m[0:26]         # Slicing returns another memoryview
   >>> m2
   <memory at 0x37f080>

El contenido de la vista se puede convertir en una cadena de bytes o
una lista de números enteros:

   >>> m2.tobytes()
   'abcdefghijklmnopqrstuvwxyz'
   >>> m2.tolist()
   [97, 98, 99, 100, 101, 102, 103, ... 121, 122]
   >>>

Los objetos "memoryview" permiten modificar el objeto subyacente si es
un objeto mutable.

   >>> m2[0] = 75
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: cannot modify read-only memory
   >>> b = bytearray(string.letters)  # Creating a mutable object
   >>> b
   bytearray(b'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ')
   >>> mb = memoryview(b)
   >>> mb[0] = '*'         # Assign to view, changing the bytearray.
   >>> b[0:5]              # The bytearray has been changed.
   bytearray(b'*bcde')
   >>>

Ver también:

  **PEP 3137** - Bytes inmutables y búfer mutable
     PEP escrito por Guido van Rossum. Implementado por Travis
     Oliphant, Antoine Pitrou y otros. Portado a 2.7 por Antoine
     Pitrou; bpo-2396.


Otros cambios de lenguaje
=========================

Algunos de los cambios mas pequeños hechos al núcleo del lenguaje de
Python son:

* La sintaxis para literales de conjuntos se ha actualizado desde
  Python 3.x. Los corchetes se utilizan para rodear el contenido del
  conjunto mutable resultante; los literales de conjunto se distinguen
  de los diccionarios por no contener dos puntos ni valores. "{}"
  sigue representando un diccionario vacío; use "set()" para un
  conjunto vacío.

     >>> {1, 2, 3, 4, 5}
     set([1, 2, 3, 4, 5])
     >>> set() # empty set
     set([])
     >>> {}    # empty dict
     {}

  Portado por Alexandre Vassalotti; bpo-2335.

* El diccionario y las comprensiones de conjuntos son otra
  característica portada desde 3.x, que generaliza las comprensiones
  de listas / generadores para usar la sintaxis literal para conjuntos
  y diccionarios.

     >>> {x: x*x for x in range(6)}
     {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
     >>> {('a'*x) for x in range(6)}
     set(['', 'a', 'aa', 'aaa', 'aaaa', 'aaaaa'])

  Portado por Alexandre Vassalotti; bpo-2333.

* La declaración "with" ahora puede usar múltiples administradores de
  contexto en una declaración. Los administradores de contexto se
  procesan de izquierda a derecha y cada uno se trata como si
  comenzara una nueva declaración "with". Esto significa que:

     with A() as a, B() as b:
         ... suite of statements ...

  es equivalente a:

     with A() as a:
         with B() as b:
             ... suite of statements ...

  La función "contextlib.nested()" provee una función muy similar, por
  lo que ya no es necesario y ha quedado obsoleto.

  (Propuesta en https://codereview.appspot.com/53094; implementado por
  Georg Brandl.)

* Las conversiones entre números de punto flotante y cadenas ahora se
  redondean correctamente en la mayoría de las plataformas. Estas
  conversiones ocurren en muchos lugares diferentes: "str()" en
  flotantes y números complejos; los constructores "float" y
  "complex"; formato numérico; serializar y deserializar flotantes y
  números complejos usando los módulos "marshal", "pickle" y "json";
  análisis de literales flotantes e imaginarios en código Python; y
  conversión "Decimal" a flotante.

  Relacionado con esto, el "repr()" de un número de punto flotante *x*
  ahora devuelve un resultado basado en la cadena decimal más corta
  que se garantiza que se redondeará a *x* con el redondeo correcto
  (con el modo de redondeo *round-half-to-even*). Anteriormente, daba
  una cadena basada en redondear x a 17 dígitos decimales.

  La biblioteca de redondeo responsable de esta mejora funciona en
  Windows y en plataformas Unix utilizando los compiladores gcc, icc o
  suncc.  Puede haber un pequeño número de plataformas donde no se
  puede garantizar el correcto funcionamiento de este código, por lo
  que el código no se utiliza en dichos sistemas.  Puede averiguar qué
  código se está utilizando marcando "sys.float_repr_style", que será
  "short" si el nuevo código está en uso y "legacy" si no lo está.

  Implementado por Eric Smith y Mark Dickinson, utilizando la
  biblioteca de David Gay's "dtoa.c"; bpo-7117.

* Las conversiones de enteros largos y enteros normales a punto
  flotante ahora redondean de forma diferente, devolviendo el número
  de punto flotante más cercano al número.  Esto no importa para los
  enteros pequeños que se pueden convertir exactamente, pero para los
  números grandes que inevitablemente perderán precisión, Python 2.7
  ahora se aproxima más de cerca.  Por ejemplo, Python 2.6 calculó lo
  siguiente:

     >>> n = 295147905179352891391
     >>> float(n)
     2.9514790517935283e+20
     >>> n - long(float(n))
     65535L

  El resultado de punto flotante de Python 2.7 es mayor, pero mucho
  más cerca del valor verdadero:

     >>> n = 295147905179352891391
     >>> float(n)
     2.9514790517935289e+20
     >>> n - long(float(n))
     -1L

  (Implementado por Mark Dickinson; bpo-3166.)

  La división de enteros también es más precisa en sus comportamientos
  de redondeo. (También implementado por Mark Dickinson; bpo-1811.)

* Se ha eliminado la coerción implícita para números complejos; el
  intérprete ya no intentará llamar a un método "__coerce__()" en
  objetos complejos.  (Eliminado por Meador Inge y Mark Dickinson;
  bpo-5211.)

* El método "str.format()" ahora admite la numeración automática de
  los campos de reemplazo.  Esto hace que el uso de "str.format()" se
  parezca más al uso del formato "%s":

     >>> '{}:{}:{}'.format(2009, 04, 'Sunday')
     '2009:4:Sunday'
     >>> '{}:{}:{day}'.format(2009, 4, day='Sunday')
     '2009:4:Sunday'

  La numeración automática toma los campos de izquierda a derecha, por
  lo que el primer especificador "{...}" usará el primer argumento a
  "str.format()", el siguiente especificador usará el siguiente
  argumento, y así sucesivamente.  No puede mezclar la numeración
  automática y la numeración explícita -- o numeran todos los campos
  especificados o ninguno de ellos -- pero puede mezclar la numeración
  automática y los campos con nombre, como en el segundo ejemplo
  anterior.  (Contribuido por Eric Smith; bpo-5237.)

  Los números complejos ahora admiten correctamente el uso con
  "format()" y, por defecto, están alineados a la derecha. Especificar
  una precisión o separación de comas se aplica a las partes reales e
  imaginarias del número, pero un ancho de campo y una alineación
  especificados se aplica a la totalidad de la salida ''1.5+3j''
  resultante.  (Contribuido por Eric Smith; bpo-1588 y bpo-7988.)

  El código de formato 'F' ahora siempre da formato a su salida
  utilizando caracteres en mayúsculas, por lo que ahora producirá
  'INF' y 'NAN'. (Contribuido por Eric Smith; bpo-3382.)

  Un cambio de bajo nivel: el método "object.__format__()" ahora
  desencadena un "PendingDeprecationWarning" si se pasa una cadena de
  formato, porque el método "__format__()" para "object" convierte el
  objeto en una representación de cadena y da formato a eso.
  Anteriormente, el método aplicaba silenciosamente la cadena de
  formato a la representación de cadena, pero eso podía ocultar
  errores en el código de Python.  Si proporciona información de
  formato, como una alineación o una precisión, presumiblemente espera
  que el formato se aplique de alguna manera específica del objeto.
  (Arreglado por Eric Smith; bpo-7994.)

* Los tipos "int()" y "long()" obtuvieron un método "bit_length" que
  devuelve el número de bits necesarios para representar su argumento
  en binario:

     >>> n = 37
     >>> bin(n)
     '0b100101'
     >>> n.bit_length()
     6
     >>> n = 2**123-1
     >>> n.bit_length()
     123
     >>> (n+1).bit_length()
     124

  (Contribuido por Fredrik Johansson y Victor Stinner; bpo-3439.)

* La instrucción "import" ya no intentará una importación absoluta si
  se produce un error en una importación relativa (por ejemplo, "from
  .os import sep").  Esto corrige un error, pero posiblemente podría
  romper ciertas instrucciones "import" que solo funcionaban por
  accidente.  (Arreglado por Meador Inge; bpo-7902.)

* Ahora es posible que una subclase del tipo integrado "unicode"
  reemplace el método "__unicode__()".  (Implementado por Victor
  Stinner; bpo-1583863.)

* El método "translate()" del tipo "bytearray" ahora acepta "None"
  como su primer argumento.  (Arreglado por Georg Brandl; bpo-4759.)

* Cuando se usa "@classmethod" y "@staticmethod" para envolver métodos
  como clase o métodos estáticos, el objeto decorador ahora expone la
  función decoradora como su atributo "__func__". (Contribuido por
  Amaury Forgeot d'Arc, después de una sugerencia de George Sakkis;
  bpo-5982.)

* Cuando se estableció un conjunto restringido de atributos mediante
  "__slots__", eliminar un atributo desarmado no llamaría
  "AttributeError" como cabría esperar.  Corregido por Benjamin
  Peterson; bpo-7604.)

* Ahora se admiten dos nuevas codificaciones: "cp720", que se utiliza
  principalmente para texto árabe; y "cp858", una variante de CP 850
  que añade el símbolo del euro.  (CP720 contribuido por Alexander
  Belchenko y Amaury Forgeot d'Arc en bpo-1616979; CP858 contribuido
  por Tim Hatch en bpo-8016.)

* El objeto "file" ahora establecerá el atributo "filename" en la
  excepción "IOError" al intentar abrir un directorio en plataformas
  POSIX (señalada por Jan Kaliszewski; bpo-4764), y ahora comprueba
  explícitamente y prohíbe escribir en objetos de archivo de solo
  lectura en lugar de confiar en la biblioteca C para detectar e
  informar del error (arreglado por Stefan Krah; bpo-5677).

* El tokenizador de Python ahora traduce las terminaciones de línea
  por sí mismo, por lo que la función integrada "compile()" ahora
  acepta código mediante cualquier convención de fin de línea.
  Además, ya no requiere que el código finalice en una nueva línea.

* Los paréntesis adicionales en las definiciones de función son
  ilegales en Python 3.x, lo que significa que se obtiene un error de
  sintaxis de "def f((x)): pass".  En el modo de advertencia Python3,
  Python 2.7 ahora advertirá sobre este uso no común. (Anotado por
  James Lingard; :issue:'7362'.)

* Ahora es posible crear referencias débiles a objetos de clase de
  estilo antiguo.  Las clases de estilo nuevo siempre eran de
  referencia débil.  (Corregido por Antoine Pitrou; bpo-8268.)

* Cuando se recicla un objeto de módulo, el diccionario del módulo
  ahora solo se borra si nadie más tiene una referencia al diccionario
  (bpo-7140).


Cambios en el intérprete
------------------------

Una nueva variable de entorno, "PYTHONWARNINGS", permite controlar las
advertencias.  Debe establecerse en una cadena que contenga la
configuración de advertencia, equivalente a las utilizadas con el
modificador "-W", separado por comas. (Contribuido por Brian Curtin;
bpo-7301.)

Por ejemplo, la siguiente configuración imprimirá advertencias cada
vez que se produzcan, pero convertirá las advertencias del módulo
"Cookie" en un error.  (La sintaxis exacta para establecer una
variable de entorno varía entre sistemas operativos y vaciados.)

   export PYTHONWARNINGS=all,error:::Cookie:0


Optimizaciones
--------------

Se han añadido varias mejoras de rendimiento:

* Se agregó un nuevo código de operación para realizar la
  configuración inicial de las instrucciones "with", buscando los
  métodos "__enter__()" y "__exit__()".  (Contribución de Benjamin
  Peterson.)

* El recolector de elementos no utilizados ahora funciona mejor para
  un patrón de uso común: cuando se asignan muchos objetos sin
  desasignar ninguno de ellos.  Esto anteriormente tomaría tiempo
  cuadrático para la recolección de elementos no utilizados, pero
  ahora el número de colecciones de elementos no utilizados completas
  se reduce a medida que crece el número de objetos en el montón. La
  nueva lógica solo realiza un paso completo de recolección de
  elementos no utilizados cuando la generación media se ha recopilado
  10 veces y cuando el número de objetos supervivientes de la
  generación media supera el 10% del número de objetos de la
  generación más antigua.  (Sugerido por Martin von Löwis e
  implementado por Antoine Pitrou; bpo-4074.)

* The garbage collector tries to avoid tracking simple containers
  which can't be part of a cycle. In Python 2.7, this is now true for
  tuples and dicts containing atomic types (such as ints, strings,
  etc.). Transitively, a dict containing tuples of atomic types won't
  be tracked either. This helps reduce the cost of each garbage
  collection by decreasing the number of objects to be considered and
  traversed by the collector. (Contributed by Antoine Pitrou;
  bpo-4688.)

* Long integers are now stored internally either in base "2**15" or in
  base "2**30", the base being determined at build time.  Previously,
  they were always stored in base "2**15".  Using base "2**30" gives
  significant performance improvements on 64-bit machines, but
  benchmark results on 32-bit machines have been mixed.  Therefore,
  the default is to use base "2**30" on 64-bit machines and base
  "2**15" on 32-bit machines; on Unix, there's a new configure option
  "--enable-big-digits" that can be used to override this default.

  Apart from the performance improvements this change should be
  invisible to end users, with one exception: for testing and
  debugging purposes there's a new structseq "sys.long_info" that
  provides information about the internal format, giving the number of
  bits per digit and the size in bytes of the C type used to store
  each digit:

     >>> import sys
     >>> sys.long_info
     sys.long_info(bits_per_digit=30, sizeof_digit=4)

  (Contributed by Mark Dickinson; bpo-4258.)

  Another set of changes made long objects a few bytes smaller: 2
  bytes smaller on 32-bit systems and 6 bytes on 64-bit. (Contributed
  by Mark Dickinson; bpo-5260.)

* The division algorithm for long integers has been made faster by
  tightening the inner loop, doing shifts instead of multiplications,
  and fixing an unnecessary extra iteration. Various benchmarks show
  speedups of between 50% and 150% for long integer divisions and
  modulo operations. (Contributed by Mark Dickinson; bpo-5512.)
  Bitwise operations are also significantly faster (initial patch by
  Gregory Smith; bpo-1087418).

* The implementation of "%" checks for the left-side operand being a
  Python string and special-cases it; this results in a 1--3%
  performance increase for applications that frequently use "%" with
  strings, such as templating libraries. (Implemented by Collin
  Winter; bpo-5176.)

* List comprehensions with an "if" condition are compiled into faster
  bytecode.  (Patch by Antoine Pitrou, back-ported to 2.7 by Jeffrey
  Yasskin; bpo-4715.)

* Converting an integer or long integer to a decimal string was made
  faster by special-casing base 10 instead of using a generalized
  conversion function that supports arbitrary bases. (Patch by Gawain
  Bolton; bpo-6713.)

* The "split()", "replace()", "rindex()", "rpartition()", and
  "rsplit()" methods of string-like types (strings, Unicode strings,
  and "bytearray" objects) now use a fast reverse-search algorithm
  instead of a character-by-character scan.  This is sometimes faster
  by a factor of 10.  (Added by Florent Xicluna; bpo-7462 and
  bpo-7622.)

* The "pickle" and "cPickle" modules now automatically intern the
  strings used for attribute names, reducing memory usage of the
  objects resulting from unpickling.  (Contributed by Jake McGuire;
  bpo-5084.)

* The "cPickle" module now special-cases dictionaries, nearly halving
  the time required to pickle them. (Contributed by Collin Winter;
  bpo-5670.)


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 "bdb" module's base debugging class "Bdb" gained a feature for
  skipping modules.  The constructor now takes an iterable containing
  glob-style patterns such as "django.*"; the debugger will not step
  into stack frames from a module that matches one of these patterns.
  (Contributed by Maru Newby after a suggestion by Senthil Kumaran;
  bpo-5142.)

* The "binascii" module now supports the buffer API, so it can be used
  with "memoryview" instances and other similar buffer objects.
  (Backported from 3.x by Florent Xicluna; bpo-7703.)

* Updated module: the "bsddb" module has been updated from 4.7.2devel9
  to version 4.8.4 of the pybsddb package. The new version features
  better Python 3.x compatibility, various bug fixes, and adds several
  new BerkeleyDB flags and methods. (Updated by Jesús Cea Avión;
  bpo-8156.  The pybsddb changelog can be read at
  http://hg.jcea.es/pybsddb/file/tip/ChangeLog.)

* The "bz2" module's "BZ2File" now supports the context management
  protocol, so you can write "with bz2.BZ2File(...) as f:".
  (Contributed by Hagen Fürstenau; bpo-3860.)

* New class: the "Counter" class in the "collections" module is useful
  for tallying data.  "Counter" instances behave mostly like
  dictionaries but return zero for missing keys instead of raising a
  "KeyError":

     >>> from collections import Counter
     >>> c = Counter()
     >>> for letter in 'here is a sample of english text':
     ...   c[letter] += 1
     ...
     >>> c 
     Counter({' ': 6, 'e': 5, 's': 3, 'a': 2, 'i': 2, 'h': 2,
     'l': 2, 't': 2, 'g': 1, 'f': 1, 'm': 1, 'o': 1, 'n': 1,
     'p': 1, 'r': 1, 'x': 1})
     >>> c['e']
     5
     >>> c['z']
     0

  There are three additional "Counter" methods. "most_common()"
  returns the N most common elements and their counts.  "elements()"
  returns an iterator over the contained elements, repeating each
  element as many times as its count. "subtract()" takes an iterable
  and subtracts one for each element instead of adding; if the
  argument is a dictionary or another "Counter", the counts are
  subtracted.

     >>> c.most_common(5)
     [(' ', 6), ('e', 5), ('s', 3), ('a', 2), ('i', 2)]
     >>> c.elements() ->
        'a', 'a', ' ', ' ', ' ', ' ', ' ', ' ',
        'e', 'e', 'e', 'e', 'e', 'g', 'f', 'i', 'i',
        'h', 'h', 'm', 'l', 'l', 'o', 'n', 'p', 's',
        's', 's', 'r', 't', 't', 'x'
     >>> c['e']
     5
     >>> c.subtract('very heavy on the letter e')
     >>> c['e']    # Count is now lower
     -1

  Contributed by Raymond Hettinger; bpo-1696199.

  New class: "OrderedDict" is described in the earlier section PEP
  372: Adición de un diccionario ordenado a las colecciones.

  New method: The "deque" data type now has a "count()" method that
  returns the number of contained elements equal to the supplied
  argument *x*, and a "reverse()" method that reverses the elements of
  the deque in-place.  "deque" also exposes its maximum length as the
  read-only "maxlen" attribute. (Both features added by Raymond
  Hettinger.)

  The "namedtuple" class now has an optional *rename* parameter. If
  *rename* is true, field names that are invalid because they've been
  repeated or aren't legal Python identifiers will be renamed to legal
  names that are derived from the field's position within the list of
  fields:

  >>> from collections import namedtuple
  >>> T = namedtuple('T', ['field1', '$illegal', 'for', 'field2'], rename=True)
  >>> T._fields
  ('field1', '_1', '_2', 'field2')

  (Added by Raymond Hettinger; bpo-1818.)

  Finally, the "Mapping" abstract base class now returns
  "NotImplemented" if a mapping is compared to another type that isn't
  a "Mapping". (Fixed by Daniel Stutzbach; bpo-8729.)

* Constructors for the parsing classes in the "ConfigParser" module
  now take an *allow_no_value* parameter, defaulting to false; if
  true, options without values will be allowed.  For example:

     >>> import ConfigParser, StringIO
     >>> sample_config = """
     ... [mysqld]
     ... user = mysql
     ... pid-file = /var/run/mysqld/mysqld.pid
     ... skip-bdb
     ... """
     >>> config = ConfigParser.RawConfigParser(allow_no_value=True)
     >>> config.readfp(StringIO.StringIO(sample_config))
     >>> config.get('mysqld', 'user')
     'mysql'
     >>> print config.get('mysqld', 'skip-bdb')
     None
     >>> print config.get('mysqld', 'unknown')
     Traceback (most recent call last):
       ...
     NoOptionError: No option 'unknown' in section: 'mysqld'

  (Contributed by Mats Kindahl; bpo-7005.)

* Deprecated function: "contextlib.nested()", which allows handling
  more than one context manager with a single "with" statement, has
  been deprecated, because the "with" statement now supports multiple
  context managers.

* The "cookielib" module now ignores cookies that have an invalid
  version field, one that doesn't contain an integer value.  (Fixed by
  John J. Lee; bpo-3924.)

* The "copy" module's "deepcopy()" function will now correctly copy
  bound instance methods.  (Implemented by Robert Collins; bpo-1515.)

* The "ctypes" module now always converts "None" to a C "NULL" pointer
  for arguments declared as pointers.  (Changed by Thomas Heller;
  bpo-4606.)  The underlying libffi library has been updated to
  version 3.0.9, containing various fixes for different platforms.
  (Updated by Matthias Klose; bpo-8142.)

* New method: the "datetime" module's "timedelta" class gained a
  "total_seconds()" method that returns the number of seconds in the
  duration.  (Contributed by Brian Quinlan; bpo-5788.)

* New method: the "Decimal" class gained a "from_float()" class method
  that performs an exact conversion of a floating-point number to a
  "Decimal". This exact conversion strives for the closest decimal
  approximation to the floating-point representation's value; the
  resulting decimal value will therefore still include the inaccuracy,
  if any. For example, "Decimal.from_float(0.1)" returns "Decimal('0.
  1000000000000000055511151231257827021181583404541015625')".
  (Implemented by Raymond Hettinger; bpo-4796.)

  Comparing instances of "Decimal" with floating-point numbers now
  produces sensible results based on the numeric values of the
  operands.  Previously such comparisons would fall back to Python's
  default rules for comparing objects, which produced arbitrary
  results based on their type.  Note that you still cannot combine
  "Decimal" and floating-point in other operations such as addition,
  since you should be explicitly choosing how to convert between float
  and "Decimal".  (Fixed by Mark Dickinson; bpo-2531.)

  The constructor for "Decimal" now accepts floating-point numbers
  (added by Raymond Hettinger; bpo-8257) and non-European Unicode
  characters such as Arabic-Indic digits (contributed by Mark
  Dickinson; bpo-6595).

  Most of the methods of the "Context" class now accept integers as
  well as "Decimal" instances; the only exceptions are the
  "canonical()" and "is_canonical()" methods.  (Patch by Juan José
  Conti; bpo-7633.)

  When using "Decimal" instances with a string's "format()" method,
  the default alignment was previously left-alignment.  This has been
  changed to right-alignment, which is more sensible for numeric
  types.  (Changed by Mark Dickinson; bpo-6857.)

  Comparisons involving a signaling NaN value (or "sNAN") now signal
  "InvalidOperation" instead of silently returning a true or false
  value depending on the comparison operator.  Quiet NaN values (or
  "NaN") are now hashable.  (Fixed by Mark Dickinson; bpo-7279.)

* The "difflib" module now produces output that is more compatible
  with modern **diff**/**patch** tools through one small change, using
  a tab character instead of spaces as a separator in the header
  giving the filename.  (Fixed by Anatoly Techtonik; bpo-7585.)

* The Distutils "sdist" command now always regenerates the "MANIFEST"
  file, since even if the "MANIFEST.in" or "setup.py" files haven't
  been modified, the user might have created some new files that
  should be included. (Fixed by Tarek Ziadé; bpo-8688.)

* The "doctest" module's "IGNORE_EXCEPTION_DETAIL" flag will now
  ignore the name of the module containing the exception being tested.
  (Patch by Lennart Regebro; bpo-7490.)

* The "email" module's "Message" class will now accept a Unicode-
  valued payload, automatically converting the payload to the encoding
  specified by "output_charset". (Added by R. David Murray;
  bpo-1368247.)

* The "Fraction" class now accepts a single float or "Decimal"
  instance, or two rational numbers, as arguments to its constructor.
  (Implemented by Mark Dickinson; rationals added in bpo-5812, and
  float/decimal in bpo-8294.)

  Ordering comparisons ("<", "<=", ">", ">=") between fractions and
  complex numbers now raise a "TypeError". This fixes an oversight,
  making the "Fraction" match the other numeric types.

* New class: "FTP_TLS" in the "ftplib" module provides secure FTP
  connections using TLS encapsulation of authentication as well as
  subsequent control and data transfers. (Contributed by Giampaolo
  Rodola; bpo-2054.)

  The "storbinary()" method for binary uploads can now restart uploads
  thanks to an added *rest* parameter (patch by Pablo Mouzo;
  bpo-6845.)

* New class decorator: "total_ordering()" in the "functools" module
  takes a class that defines an "__eq__()" method and one of
  "__lt__()", "__le__()", "__gt__()", or "__ge__()", and generates the
  missing comparison methods.  Since the "__cmp__()" method is being
  deprecated in Python 3.x, this decorator makes it easier to define
  ordered classes. (Added by Raymond Hettinger; bpo-5479.)

  New function: "cmp_to_key()" will take an old-style comparison
  function that expects two arguments and return a new callable that
  can be used as the *key* parameter to functions such as "sorted()",
  "min()" and "max()", etc.  The primary intended use is to help with
  making code compatible with Python 3.x. (Added by Raymond
  Hettinger.)

* New function: the "gc" module's "is_tracked()" returns true if a
  given instance is tracked by the garbage collector, false otherwise.
  (Contributed by Antoine Pitrou; bpo-4688.)

* The "gzip" module's "GzipFile" now supports the context management
  protocol, so you can write "with gzip.GzipFile(...) as f:"
  (contributed by Hagen Fürstenau; bpo-3860), and it now implements
  the "io.BufferedIOBase" ABC, so you can wrap it with
  "io.BufferedReader" for faster processing (contributed by Nir Aides;
  bpo-7471). It's also now possible to override the modification time
  recorded in a gzipped file by providing an optional timestamp to the
  constructor.  (Contributed by Jacques Frechet; bpo-4272.)

  Files in gzip format can be padded with trailing zero bytes; the
  "gzip" module will now consume these trailing bytes.  (Fixed by
  Tadek Pietraszek and Brian Curtin; bpo-2846.)

* New attribute: the "hashlib" module now has an "algorithms"
  attribute containing a tuple naming the supported algorithms. In
  Python 2.7, "hashlib.algorithms" contains "('md5', 'sha1', 'sha224',
  'sha256', 'sha384', 'sha512')". (Contributed by Carl Chenet;
  bpo-7418.)

* The default "HTTPResponse" class used by the "httplib" module now
  supports buffering, resulting in much faster reading of HTTP
  responses. (Contributed by Kristján Valur Jónsson; bpo-4879.)

  The "HTTPConnection" and "HTTPSConnection" classes now support a
  *source_address* parameter, a "(host, port)" 2-tuple giving the
  source address that will be used for the connection. (Contributed by
  Eldon Ziegler; bpo-3972.)

* The "ihooks" module now supports relative imports.  Note that
  "ihooks" is an older module for customizing imports, superseded by
  the "imputil" module added in Python 2.0. (Relative import support
  added by Neil Schemenauer.)

* The "imaplib" module now supports IPv6 addresses. (Contributed by
  Derek Morr; bpo-1655.)

* New function: the "inspect" module's "getcallargs()" takes a
  callable and its positional and keyword arguments, and figures out
  which of the callable's parameters will receive each argument,
  returning a dictionary mapping argument names to their values.  For
  example:

     >>> from inspect import getcallargs
     >>> def f(a, b=1, *pos, **named):
     ...     pass
     >>> getcallargs(f, 1, 2, 3)
     {'a': 1, 'b': 2, 'pos': (3,), 'named': {}}
     >>> getcallargs(f, a=2, x=4)
     {'a': 2, 'b': 1, 'pos': (), 'named': {'x': 4}}
     >>> getcallargs(f)
     Traceback (most recent call last):
     ...
     TypeError: f() takes at least 1 argument (0 given)

  Contributed by George Sakkis; bpo-3135.

* Updated module: The "io" library has been upgraded to the version
  shipped with Python 3.1.  For 3.1, the I/O library was entirely
  rewritten in C and is 2 to 20 times faster depending on the task
  being performed.  The original Python version was renamed to the
  "_pyio" module.

  One minor resulting change: the "io.TextIOBase" class now has an
  "errors" attribute giving the error setting used for encoding and
  decoding errors (one of "'strict'", "'replace'", "'ignore'").

  The "io.FileIO" class now raises an "OSError" when passed an invalid
  file descriptor.  (Implemented by Benjamin Peterson; bpo-4991.)  The
  "truncate()" method now preserves the file position; previously it
  would change the file position to the end of the new file.  (Fixed
  by Pascal Chambon; bpo-6939.)

* New function: "itertools.compress(data, selectors)" takes two
  iterators.  Elements of *data* are returned if the corresponding
  value in *selectors* is true:

     itertools.compress('ABCDEF', [1,0,1,0,1,1]) =>
       A, C, E, F

  New function: "itertools.combinations_with_replacement(iter, r)"
  returns all the possible *r*-length combinations of elements from
  the iterable *iter*.  Unlike "combinations()", individual elements
  can be repeated in the generated combinations:

     itertools.combinations_with_replacement('abc', 2) =>
       ('a', 'a'), ('a', 'b'), ('a', 'c'),
       ('b', 'b'), ('b', 'c'), ('c', 'c')

  Note that elements are treated as unique depending on their position
  in the input, not their actual values.

  The "itertools.count()" function now has a *step* argument that
  allows incrementing by values other than 1.  "count()" also now
  allows keyword arguments, and using non-integer values such as
  floats or "Decimal" instances.  (Implemented by Raymond Hettinger;
  bpo-5032.)

  "itertools.combinations()" and "itertools.product()" previously
  raised "ValueError" for values of *r* larger than the input
  iterable.  This was deemed a specification error, so they now return
  an empty iterator.  (Fixed by Raymond Hettinger; bpo-4816.)

* Updated module: The "json" module was upgraded to version 2.0.9 of
  the simplejson package, which includes a C extension that makes
  encoding and decoding faster. (Contributed by Bob Ippolito;
  bpo-4136.)

  To support the new "collections.OrderedDict" type, "json.load()" now
  has an optional *object_pairs_hook* parameter that will be called
  with any object literal that decodes to a list of pairs.
  (Contributed by Raymond Hettinger; bpo-5381.)

* The "mailbox" module's "Maildir" class now records the timestamp on
  the directories it reads, and only re-reads them if the modification
  time has subsequently changed.  This improves performance by
  avoiding unneeded directory scans.  (Fixed by A.M. Kuchling and
  Antoine Pitrou; bpo-1607951, bpo-6896.)

* New functions: the "math" module gained "erf()" and "erfc()" for the
  error function and the complementary error function, "expm1()" which
  computes "e**x - 1" with more precision than using "exp()" and
  subtracting 1, "gamma()" for the Gamma function, and "lgamma()" for
  the natural log of the Gamma function. (Contributed by Mark
  Dickinson and nirinA raseliarison; bpo-3366.)

* The "multiprocessing" module's "Manager*" classes can now be passed
  a callable that will be called whenever a subprocess is started,
  along with a set of arguments that will be passed to the callable.
  (Contributed by lekma; bpo-5585.)

  The "Pool" class, which controls a pool of worker processes, now has
  an optional *maxtasksperchild* parameter.  Worker processes will
  perform the specified number of tasks and then exit, causing the
  "Pool" to start a new worker.  This is useful if tasks may leak
  memory or other resources, or if some tasks will cause the worker to
  become very large. (Contributed by Charles Cazabon; bpo-6963.)

* The "nntplib" module now supports IPv6 addresses. (Contributed by
  Derek Morr; bpo-1664.)

* New functions: the "os" module wraps the following POSIX system
  calls: "getresgid()" and "getresuid()", which return the real,
  effective, and saved GIDs and UIDs; "setresgid()" and "setresuid()",
  which set real, effective, and saved GIDs and UIDs to new values;
  "initgroups()", which initialize the group access list for the
  current process.  (GID/UID functions contributed by Travis H.;
  bpo-6508.  Support for initgroups added by Jean-Paul Calderone;
  bpo-7333.)

  The "os.fork()" function now re-initializes the import lock in the
  child process; this fixes problems on Solaris when "fork()" is
  called from a thread.  (Fixed by Zsolt Cserna; bpo-7242.)

* In the "os.path" module, the "normpath()" and "abspath()" functions
  now preserve Unicode; if their input path is a Unicode string, the
  return value is also a Unicode string. ("normpath()" fixed by Matt
  Giuca in bpo-5827; "abspath()" fixed by Ezio Melotti in bpo-3426.)

* The "pydoc" module now has help for the various symbols that Python
  uses.  You can now do "help('<<')" or "help('@')", for example.
  (Contributed by David Laban; bpo-4739.)

* The "re" module's "split()", "sub()", and "subn()" now accept an
  optional *flags* argument, for consistency with the other functions
  in the module.  (Added by Gregory P. Smith.)

* New function: "run_path()" in the "runpy" module will execute the
  code at a provided *path* argument.  *path* can be the path of a
  Python source file ("example.py"), a compiled bytecode file
  ("example.pyc"), a directory ("./package/"), or a zip archive
  ("example.zip").  If a directory or zip path is provided, it will be
  added to the front of "sys.path" and the module "__main__" will be
  imported.  It's expected that the directory or zip contains a
  "__main__.py"; if it doesn't, some other "__main__.py" might be
  imported from a location later in "sys.path".  This makes more of
  the machinery of "runpy" available to scripts that want to mimic the
  way Python's command line processes an explicit path name. (Added by
  Nick Coghlan; bpo-6816.)

* New function: in the "shutil" module, "make_archive()" takes a
  filename, archive type (zip or tar-format), and a directory path,
  and creates an archive containing the directory's contents. (Added
  by Tarek Ziadé.)

  "shutil"'s "copyfile()" and "copytree()" functions now raise a
  "SpecialFileError" exception when asked to copy a named pipe.
  Previously the code would treat named pipes like a regular file by
  opening them for reading, and this would block indefinitely.  (Fixed
  by Antoine Pitrou; bpo-3002.)

* The "signal" module no longer re-installs the signal handler unless
  this is truly necessary, which fixes a bug that could make it
  impossible to catch the EINTR signal robustly.  (Fixed by Charles-
  Francois Natali; bpo-8354.)

* New functions: in the "site" module, three new functions return
  various site- and user-specific paths. "getsitepackages()" returns a
  list containing all global site-packages directories,
  "getusersitepackages()" returns the path of the user's site-packages
  directory, and "getuserbase()" returns the value of the "USER_BASE"
  environment variable, giving the path to a directory that can be
  used to store data. (Contributed by Tarek Ziadé; bpo-6693.)

  The "site" module now reports exceptions occurring when the
  "sitecustomize" module is imported, and will no longer catch and
  swallow the "KeyboardInterrupt" exception.  (Fixed by Victor
  Stinner; bpo-3137.)

* The "create_connection()" function gained a *source_address*
  parameter, a "(host, port)" 2-tuple giving the source address that
  will be used for the connection. (Contributed by Eldon Ziegler;
  bpo-3972.)

  The "recv_into()" and "recvfrom_into()" methods will now write into
  objects that support the buffer API, most usefully the "bytearray"
  and "memoryview" objects.  (Implemented by Antoine Pitrou;
  bpo-8104.)

* The "SocketServer" module's "TCPServer" class now supports socket
  timeouts and disabling the Nagle algorithm. The
  "disable_nagle_algorithm" class attribute defaults to "False"; if
  overridden to be true, new request connections will have the
  TCP_NODELAY option set to prevent buffering many small sends into a
  single TCP packet. The "timeout" class attribute can hold a timeout
  in seconds that will be applied to the request socket; if no request
  is received within that time, "handle_timeout()" will be called and
  "handle_request()" will return. (Contributed by Kristján Valur
  Jónsson; bpo-6192 and bpo-6267.)

* Updated module: the "sqlite3" module has been updated to version
  2.6.0 of the pysqlite package. Version 2.6.0 includes a number of
  bugfixes, and adds the ability to load SQLite extensions from shared
  libraries. Call the "enable_load_extension(True)" method to enable
  extensions, and then call "load_extension()" to load a particular
  shared library. (Updated by Gerhard Häring.)

* The "ssl" module's "SSLSocket" objects now support the buffer API,
  which fixed a test suite failure (fix by Antoine Pitrou; bpo-7133)
  and automatically set OpenSSL's "SSL_MODE_AUTO_RETRY", which will
  prevent an error code being returned from "recv()" operations that
  trigger an SSL renegotiation (fix by Antoine Pitrou; bpo-8222).

  The "ssl.wrap_socket()" constructor function now takes a *ciphers*
  argument that's a string listing the encryption algorithms to be
  allowed; the format of the string is described in the OpenSSL
  documentation. (Added by Antoine Pitrou; bpo-8322.)

  Another change makes the extension load all of OpenSSL's ciphers and
  digest algorithms so that they're all available.  Some SSL
  certificates couldn't be verified, reporting an "unknown algorithm"
  error.  (Reported by Beda Kosata, and fixed by Antoine Pitrou;
  bpo-8484.)

  The version of OpenSSL being used is now available as the module
  attributes "ssl.OPENSSL_VERSION" (a string),
  "ssl.OPENSSL_VERSION_INFO" (a 5-tuple), and
  "ssl.OPENSSL_VERSION_NUMBER" (an integer).  (Added by Antoine
  Pitrou; bpo-8321.)

* The "struct" module will no longer silently ignore overflow errors
  when a value is too large for a particular integer format code (one
  of "bBhHiIlLqQ"); it now always raises a "struct.error" exception.
  (Changed by Mark Dickinson; bpo-1523.)  The "pack()" function will
  also attempt to use "__index__()" to convert and pack non-integers
  before trying the "__int__()" method or reporting an error. (Changed
  by Mark Dickinson; bpo-8300.)

* New function: the "subprocess" module's "check_output()" runs a
  command with a specified set of arguments and returns the command's
  output as a string when the command runs without error, or raises a
  "CalledProcessError" exception otherwise.

     >>> subprocess.check_output(['df', '-h', '.'])
     'Filesystem     Size   Used  Avail Capacity  Mounted on\n
     /dev/disk0s2    52G    49G   3.0G    94%    /\n'

     >>> subprocess.check_output(['df', '-h', '/bogus'])
       ...
     subprocess.CalledProcessError: Command '['df', '-h', '/bogus']' returned non-zero exit status 1

  (Contributed by Gregory P. Smith.)

  The "subprocess" module will now retry its internal system calls on
  receiving an "EINTR" signal.  (Reported by several people; final
  patch by Gregory P. Smith in bpo-1068268.)

* New function: "is_declared_global()" in the "symtable" module
  returns true for variables that are explicitly declared to be
  global, false for ones that are implicitly global. (Contributed by
  Jeremy Hylton.)

* The "syslog" module will now use the value of "sys.argv[0]" as the
  identifier instead of the previous default value of "'python'".
  (Changed by Sean Reifschneider; bpo-8451.)

* The "sys.version_info" value is now a named tuple, with attributes
  named "major", "minor", "micro", "releaselevel", and "serial".
  (Contributed by Ross Light; bpo-4285.)

  "sys.getwindowsversion()" also returns a named tuple, with
  attributes named "major", "minor", "build", "platform",
  "service_pack", "service_pack_major", "service_pack_minor",
  "suite_mask", and "product_type".  (Contributed by Brian Curtin;
  bpo-7766.)

* The "tarfile" module's default error handling has changed, to no
  longer suppress fatal errors.  The default error level was
  previously 0, which meant that errors would only result in a message
  being written to the debug log, but because the debug log is not
  activated by default, these errors go unnoticed.  The default error
  level is now 1, which raises an exception if there's an error.
  (Changed by Lars Gustäbel; bpo-7357.)

  "tarfile" now supports filtering the "TarInfo" objects being added
  to a tar file.  When you call "add()", you may supply an optional
  *filter* argument that's a callable.  The *filter* callable will be
  passed the "TarInfo" for every file being added, and can modify and
  return it. If the callable returns "None", the file will be excluded
  from the resulting archive.  This is more powerful than the existing
  *exclude* argument, which has therefore been deprecated. (Added by
  Lars Gustäbel; bpo-6856.) The "TarFile" class also now supports the
  context management protocol. (Added by Lars Gustäbel; bpo-7232.)

* The "wait()" method of the "threading.Event" class now returns the
  internal flag on exit.  This means the method will usually return
  true because "wait()" is supposed to block until the internal flag
  becomes true.  The return value will only be false if a timeout was
  provided and the operation timed out. (Contributed by Tim Lesher;
  bpo-1674032.)

* The Unicode database provided by the "unicodedata" module is now
  used internally to determine which characters are numeric,
  whitespace, or represent line breaks.  The database also includes
  information from the "Unihan.txt" data file (patch by Anders
  Chrigström and Amaury Forgeot d'Arc; bpo-1571184) and has been
  updated to version 5.2.0 (updated by Florent Xicluna; bpo-8024).

* The "urlparse" module's "urlsplit()" now handles unknown URL schemes
  in a fashion compliant with **RFC 3986**: if the URL is of the form
  ""<something>://..."", the text before the "://" is treated as the
  scheme, even if it's a made-up scheme that the module doesn't know
  about.  This change may break code that worked around the old
  behaviour.  For example, Python 2.6.4 or 2.5 will return the
  following:

     >>> import urlparse
     >>> urlparse.urlsplit('invented://host/filename?query')
     ('invented', '', '//host/filename?query', '', '')

  Python 2.7 (and Python 2.6.5) will return:

     >>> import urlparse
     >>> urlparse.urlsplit('invented://host/filename?query')
     ('invented', 'host', '/filename?query', '', '')

  (Python 2.7 actually produces slightly different output, since it
  returns a named tuple instead of a standard tuple.)

  The "urlparse" module also supports IPv6 literal addresses as
  defined by **RFC 2732** (contributed by Senthil Kumaran; bpo-2987).

     >>> urlparse.urlparse('http://[1080::8:800:200C:417A]/foo')
     ParseResult(scheme='http', netloc='[1080::8:800:200C:417A]',
                 path='/foo', params='', query='', fragment='')

* New class: the "WeakSet" class in the "weakref" module is a set that
  only holds weak references to its elements; elements will be removed
  once there are no references pointing to them. (Originally
  implemented in Python 3.x by Raymond Hettinger, and backported to
  2.7 by Michael Foord.)

* The ElementTree library, "xml.etree", no longer escapes ampersands
  and angle brackets when outputting an XML processing instruction
  (which looks like "<?xml-stylesheet href="#style1"?>") or comment
  (which looks like "<!-- comment -->"). (Patch by Neil Muller;
  bpo-2746.)

* The XML-RPC client and server, provided by the "xmlrpclib" and
  "SimpleXMLRPCServer" modules, have improved performance by
  supporting HTTP/1.1 keep-alive and by optionally using gzip encoding
  to compress the XML being exchanged.  The gzip compression is
  controlled by the "encode_threshold" attribute of
  "SimpleXMLRPCRequestHandler", which contains a size in bytes;
  responses larger than this will be compressed. (Contributed by
  Kristján Valur Jónsson; bpo-6267.)

* The "zipfile" module's "ZipFile" now supports the context management
  protocol, so you can write "with zipfile.ZipFile(...) as f:".
  (Contributed by Brian Curtin; bpo-5511.)

  "zipfile" now also supports archiving empty directories and extracts
  them correctly.  (Fixed by Kuba Wieczorek; bpo-4710.) Reading files
  out of an archive is faster, and interleaving "read()" and
  "readline()" now works correctly. (Contributed by Nir Aides;
  bpo-7610.)

  The "is_zipfile()" function now accepts a file object, in addition
  to the path names accepted in earlier versions.  (Contributed by
  Gabriel Genellina; bpo-4756.)

  The "writestr()" method now has an optional *compress_type*
  parameter that lets you override the default compression method
  specified in the "ZipFile" constructor.  (Contributed by Ronald
  Oussoren; bpo-6003.)


New module: importlib
---------------------

Python 3.1 includes the "importlib" package, a re-implementation of
the logic underlying Python's "import" statement. "importlib" is
useful for implementors of Python interpreters and to users who wish
to write new importers that can participate in the import process.
Python 2.7 doesn't contain the complete "importlib" package, but
instead has a tiny subset that contains a single function,
"import_module()".

"import_module(name, package=None)" imports a module.  *name* is a
string containing the module or package's name.  It's possible to do
relative imports by providing a string that begins with a "."
character, such as "..utils.errors".  For relative imports, the
*package* argument must be provided and is the name of the package
that will be used as the anchor for the relative import.
"import_module()" both inserts the imported module into "sys.modules"
and returns the module object.

Here are some examples:

   >>> from importlib import import_module
   >>> anydbm = import_module('anydbm')  # Standard absolute import
   >>> anydbm
   <module 'anydbm' from '/p/python/Lib/anydbm.py'>
   >>> # Relative import
   >>> file_util = import_module('..file_util', 'distutils.command')
   >>> file_util
   <module 'distutils.file_util' from '/python/Lib/distutils/file_util.pyc'>

"importlib" was implemented by Brett Cannon and introduced in Python
3.1.


New module: sysconfig
---------------------

The "sysconfig" module has been pulled out of the Distutils package,
becoming a new top-level module in its own right. "sysconfig" provides
functions for getting information about Python's build process:
compiler switches, installation paths, the platform name, and whether
Python is running from its source directory.

Some of the functions in the module are:

* "get_config_var()" returns variables from Python's Makefile and the
  "pyconfig.h" file.

* "get_config_vars()" returns a dictionary containing all of the
  configuration variables.

* "get_path()" returns the configured path for a particular type of
  module: the standard library, site-specific modules, platform-
  specific modules, etc.

* "is_python_build()" returns true if you're running a binary from a
  Python source tree, and false otherwise.

Consult the "sysconfig" documentation for more details and for a
complete list of functions.

The Distutils package and "sysconfig" are now maintained by Tarek
Ziadé, who has also started a Distutils2 package (source repository at
https://hg.python.org/distutils2/) for developing a next-generation
version of Distutils.


ttk: Themed Widgets for Tk
--------------------------

Tcl/Tk 8.5 includes a set of themed widgets that re-implement basic Tk
widgets but have a more customizable appearance and can therefore more
closely resemble the native platform's widgets.  This widget set was
originally called Tile, but was renamed to Ttk (for "themed Tk") on
being added to Tcl/Tck release 8.5.

To learn more, read the "ttk" module documentation.  You may also wish
to read the Tcl/Tk manual page describing the Ttk theme engine,
available at https://www.tcl.tk/man/tcl8.5/TkCmd/ttk_intro.htm. Some
screenshots of the Python/Ttk code in use are at
https://code.google.com/archive/p/python-ttk/wikis/Screenshots.wiki.

The "ttk" module was written by Guilherme Polo and added in bpo-2983.
An alternate version called "Tile.py", written by Martin Franklin and
maintained by Kevin Walzer, was proposed for inclusion in bpo-2618,
but the authors argued that Guilherme Polo's work was more
comprehensive.


Updated module: unittest
------------------------

The "unittest" module was greatly enhanced; many new features were
added.  Most of these features were implemented by Michael Foord,
unless otherwise noted.  The enhanced version of the module is
downloadable separately for use with Python versions 2.4 to 2.6,
packaged as the "unittest2" package, from
https://pypi.org/project/unittest2.

When used from the command line, the module can automatically discover
tests.  It's not as fancy as py.test or nose, but provides a simple
way to run tests kept within a set of package directories.  For
example, the following command will search the "test/" subdirectory
for any importable test files named "test*.py":

   python -m unittest discover -s test

Consult the "unittest" module documentation for more details.
(Developed in bpo-6001.)

The "main()" function supports some other new options:

* "-b" or "--buffer" will buffer the standard output and standard
  error streams during each test.  If the test passes, any resulting
  output will be discarded; on failure, the buffered output will be
  displayed.

* "-c" or "--catch" will cause the control-C interrupt to be handled
  more gracefully.  Instead of interrupting the test process
  immediately, the currently running test will be completed and then
  the partial results up to the interruption will be reported. If
  you're impatient, a second press of control-C will cause an
  immediate interruption.

  This control-C handler tries to avoid causing problems when the code
  being tested or the tests being run have defined a signal handler of
  their own, by noticing that a signal handler was already set and
  calling it.  If this doesn't work for you, there's a
  "removeHandler()" decorator that can be used to mark tests that
  should have the control-C handling disabled.

* "-f" or "--failfast" makes test execution stop immediately when a
  test fails instead of continuing to execute further tests.
  (Suggested by Cliff Dyer and implemented by Michael Foord;
  bpo-8074.)

The progress messages now show 'x' for expected failures and 'u' for
unexpected successes when run in verbose mode. (Contributed by
Benjamin Peterson.)

Test cases can raise the "SkipTest" exception to skip a test
(bpo-1034053).

The error messages for "assertEqual()", "assertTrue()", and
"assertFalse()" failures now provide more information.  If you set the
"longMessage" attribute of your "TestCase" classes to true, both the
standard error message and any additional message you provide will be
printed for failures.  (Added by Michael Foord; bpo-5663.)

The "assertRaises()" method now returns a context handler when called
without providing a callable object to run.  For example, you can
write this:

   with self.assertRaises(KeyError):
       {}['foo']

(Implemented by Antoine Pitrou; bpo-4444.)

Module- and class-level setup and teardown fixtures are now supported.
Modules can contain "setUpModule()" and "tearDownModule()" functions.
Classes can have "setUpClass()" and "tearDownClass()" methods that
must be defined as class methods (using "@classmethod" or equivalent).
These functions and methods are invoked when the test runner switches
to a test case in a different module or class.

The methods "addCleanup()" and "doCleanups()" were added.
"addCleanup()" lets you add cleanup functions that will be called
unconditionally (after "setUp()" if "setUp()" fails, otherwise after
"tearDown()"). This allows for much simpler resource allocation and
deallocation during tests (bpo-5679).

A number of new methods were added that provide more specialized
tests.  Many of these methods were written by Google engineers for use
in their test suites; Gregory P. Smith, Michael Foord, and GvR worked
on merging them into Python's version of "unittest".

* "assertIsNone()" and "assertIsNotNone()" take one expression and
  verify that the result is or is not "None".

* "assertIs()" and "assertIsNot()" take two values and check whether
  the two values evaluate to the same object or not. (Added by Michael
  Foord; bpo-2578.)

* "assertIsInstance()" and "assertNotIsInstance()" check whether the
  resulting object is an instance of a particular class, or of one of
  a tuple of classes.  (Added by Georg Brandl; bpo-7031.)

* "assertGreater()", "assertGreaterEqual()", "assertLess()", and
  "assertLessEqual()" compare two quantities.

* "assertMultiLineEqual()" compares two strings, and if they're not
  equal, displays a helpful comparison that highlights the differences
  in the two strings.  This comparison is now used by default when
  Unicode strings are compared with "assertEqual()".

* "assertRegexpMatches()" and "assertNotRegexpMatches()" checks
  whether the first argument is a string matching or not matching the
  regular expression provided as the second argument (bpo-8038).

* "assertRaisesRegexp()" checks whether a particular exception is
  raised, and then also checks that the string representation of the
  exception matches the provided regular expression.

* "assertIn()" and "assertNotIn()" tests whether *first* is or is not
  in  *second*.

* "assertItemsEqual()" tests whether two provided sequences contain
  the same elements.

* "assertSetEqual()" compares whether two sets are equal, and only
  reports the differences between the sets in case of error.

* Similarly, "assertListEqual()" and "assertTupleEqual()" compare the
  specified types and explain any differences without necessarily
  printing their full values; these methods are now used by default
  when comparing lists and tuples using "assertEqual()". More
  generally, "assertSequenceEqual()" compares two sequences and can
  optionally check whether both sequences are of a particular type.

* "assertDictEqual()" compares two dictionaries and reports the
  differences; it's now used by default when you compare two
  dictionaries using "assertEqual()".  "assertDictContainsSubset()"
  checks whether all of the key/value pairs in *first* are found in
  *second*.

* "assertAlmostEqual()" and "assertNotAlmostEqual()" test whether
  *first* and *second* are approximately equal.  This method can
  either round their difference to an optionally-specified number of
  *places* (the default is 7) and compare it to zero, or require the
  difference to be smaller than a supplied *delta* value.

* "loadTestsFromName()" properly honors the "suiteClass" attribute of
  the "TestLoader". (Fixed by Mark Roddy; bpo-6866.)

* A new hook lets you extend the "assertEqual()" method to handle new
  data types.  The "addTypeEqualityFunc()" method takes a type object
  and a function. The function will be used when both of the objects
  being compared are of the specified type.  This function should
  compare the two objects and raise an exception if they don't match;
  it's a good idea for the function to provide additional information
  about why the two objects aren't matching, much as the new sequence
  comparison methods do.

"unittest.main()" now takes an optional "exit" argument.  If false,
"main()" doesn't call "sys.exit()", allowing "main()" to be used from
the interactive interpreter. (Contributed by J. Pablo Fernández;
bpo-3379.)

"TestResult" has new "startTestRun()" and "stopTestRun()" methods that
are called immediately before and after a test run.  (Contributed by
Robert Collins; bpo-5728.)

With all these changes, the "unittest.py" was becoming awkwardly
large, so the module was turned into a package and the code split into
several files (by Benjamin Peterson).  This doesn't affect how the
module is imported or used.

Ver también:

  http://www.voidspace.org.uk/python/articles/unittest2.shtml
     Describes the new features, how to use them, and the rationale
     for various design decisions.  (By Michael Foord.)


Updated module: ElementTree 1.3
-------------------------------

The version of the ElementTree library included with Python was
updated to version 1.3.  Some of the new features are:

* The various parsing functions now take a *parser* keyword argument
  giving an "XMLParser" instance that will be used.  This makes it
  possible to override the file's internal encoding:

     p = ET.XMLParser(encoding='utf-8')
     t = ET.XML("""<root/>""", parser=p)

  Errors in parsing XML now raise a "ParseError" exception, whose
  instances have a "position" attribute containing a (*line*,
  *column*) tuple giving the location of the problem.

* ElementTree's code for converting trees to a string has been
  significantly reworked, making it roughly twice as fast in many
  cases.  The "ElementTree.write()" and "Element.write()" methods now
  have a *method* parameter that can be "xml" (the default), "html",
  or "text".  HTML mode will output empty elements as
  "<empty></empty>" instead of "<empty/>", and text mode will skip
  over elements and only output the text chunks.  If you set the "tag"
  attribute of an element to "None" but leave its children in place,
  the element will be omitted when the tree is written out, so you
  don't need to do more extensive rearrangement to remove a single
  element.

  Namespace handling has also been improved.  All "xmlns:<whatever>"
  declarations are now output on the root element, not scattered
  throughout the resulting XML.  You can set the default namespace for
  a tree by setting the "default_namespace" attribute and can register
  new prefixes with "register_namespace()".  In XML mode, you can use
  the true/false *xml_declaration* parameter to suppress the XML
  declaration.

* New "Element" method: "extend()" appends the items from a sequence
  to the element's children.  Elements themselves behave like
  sequences, so it's easy to move children from one element to
  another:

     from xml.etree import ElementTree as ET

     t = ET.XML("""<list>
       <item>1</item> <item>2</item>  <item>3</item>
     </list>""")
     new = ET.XML('<root/>')
     new.extend(t)

     # Outputs <root><item>1</item>...</root>
     print ET.tostring(new)

* New "Element" method: "iter()" yields the children of the element as
  a generator.  It's also possible to write "for child in elem:" to
  loop over an element's children.  The existing method
  "getiterator()" is now deprecated, as is "getchildren()" which
  constructs and returns a list of children.

* New "Element" method: "itertext()" yields all chunks of text that
  are descendants of the element.  For example:

     t = ET.XML("""<list>
       <item>1</item> <item>2</item>  <item>3</item>
     </list>""")

     # Outputs ['\n  ', '1', ' ', '2', '  ', '3', '\n']
     print list(t.itertext())

* Deprecated: using an element as a Boolean (i.e., "if elem:") would
  return true if the element had any children, or false if there were
  no children.  This behaviour is confusing -- "None" is false, but so
  is a childless element? -- so it will now trigger a "FutureWarning".
  In your code, you should be explicit: write "len(elem) != 0" if
  you're interested in the number of children, or "elem is not None".

Fredrik Lundh develops ElementTree and produced the 1.3 version; you
can read his article describing 1.3 at https://web.archive.org/web/20
200703234532/http://effbot.org/zone/elementtree-13-intro.htm. Florent
Xicluna updated the version included with Python, after discussions on
python-dev and in bpo-6472.)


Build and C API Changes
=======================

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

* The latest release of the GNU Debugger, GDB 7, can be scripted using
  Python. When you begin debugging an executable program P, GDB will
  look for a file named "P-gdb.py" and automatically read it.  Dave
  Malcolm contributed a "python-gdb.py" that adds a number of commands
  useful when debugging Python itself.  For example, "py-up" and "py-
  down" go up or down one Python stack frame, which usually
  corresponds to several C stack frames.  "py-print" prints the value
  of a Python variable, and "py-bt" prints the Python stack trace.
  (Added as a result of bpo-8032.)

* If you use the ".gdbinit" file provided with Python, the "pyo" macro
  in the 2.7 version now works correctly when the thread being
  debugged doesn't hold the GIL; the macro now acquires it before
  printing. (Contributed by Victor Stinner; bpo-3632.)

* "Py_AddPendingCall()" is now thread-safe, letting any worker thread
  submit notifications to the main Python thread.  This is
  particularly useful for asynchronous IO operations. (Contributed by
  Kristján Valur Jónsson; bpo-4293.)

* New function: "PyCode_NewEmpty()" creates an empty code object; only
  the filename, function name, and first line number are required.
  This is useful for extension modules that are attempting to
  construct a more useful traceback stack.  Previously such extensions
  needed to call "PyCode_New()", which had many more arguments.
  (Added by Jeffrey Yasskin.)

* New function: "PyErr_NewExceptionWithDoc()" creates a new exception
  class, just as the existing "PyErr_NewException()" does, but takes
  an extra "char *" argument containing the docstring for the new
  exception class.  (Added by 'lekma' on the Python bug tracker;
  bpo-7033.)

* New function: "PyFrame_GetLineNumber()" takes a frame object and
  returns the line number that the frame is currently executing.
  Previously code would need to get the index of the bytecode
  instruction currently executing, and then look up the line number
  corresponding to that address.  (Added by Jeffrey Yasskin.)

* New functions: "PyLong_AsLongAndOverflow()" and
  "PyLong_AsLongLongAndOverflow()"  approximates a Python long integer
  as a C "long" or "long long". If the number is too large to fit into
  the output type, an *overflow* flag is set and returned to the
  caller. (Contributed by Case Van Horsen; bpo-7528 and bpo-7767.)

* New function: stemming from the rewrite of string-to-float
  conversion, a new "PyOS_string_to_double()" function was added.  The
  old "PyOS_ascii_strtod()" and "PyOS_ascii_atof()" functions are now
  deprecated.

* New function: "PySys_SetArgvEx()" sets the value of "sys.argv" and
  can optionally update "sys.path" to include the directory containing
  the script named by "sys.argv[0]" depending on the value of an
  *updatepath* parameter.

  This function was added to close a security hole for applications
  that embed Python.  The old function, "PySys_SetArgv()", would
  always update "sys.path", and sometimes it would add the current
  directory.  This meant that, if you ran an application embedding
  Python in a directory controlled by someone else, attackers could
  put a Trojan-horse module in the directory (say, a file named
  "os.py") that your application would then import and run.

  If you maintain a C/C++ application that embeds Python, check
  whether you're calling "PySys_SetArgv()" and carefully consider
  whether the application should be using "PySys_SetArgvEx()" with
  *updatepath* set to false.

  Security issue reported as CVE-2008-5983; discussed in bpo-5753, and
  fixed by Antoine Pitrou.

* New macros: the Python header files now define the following macros:
  "Py_ISALNUM", "Py_ISALPHA", "Py_ISDIGIT", "Py_ISLOWER",
  "Py_ISSPACE", "Py_ISUPPER", "Py_ISXDIGIT", "Py_TOLOWER", and
  "Py_TOUPPER". All of these functions are analogous to the C standard
  macros for classifying characters, but ignore the current locale
  setting, because in several places Python needs to analyze
  characters in a locale-independent way.  (Added by Eric Smith;
  bpo-5793.)

* Removed function: "PyEval_CallObject" is now only available as a
  macro.  A function version was being kept around to preserve ABI
  linking compatibility, but that was in 1997; it can certainly be
  deleted by now.  (Removed by Antoine Pitrou; bpo-8276.)

* New format codes: the "PyFormat_FromString()",
  "PyFormat_FromStringV()", and "PyErr_Format()" functions now accept
  "%lld" and "%llu" format codes for displaying C's "long long" types.
  (Contributed by Mark Dickinson; bpo-7228.)

* The complicated interaction between threads and process forking has
  been changed.  Previously, the child process created by "os.fork()"
  might fail because the child is created with only a single thread
  running, the thread performing the "os.fork()". If other threads
  were holding a lock, such as Python's import lock, when the fork was
  performed, the lock would still be marked as "held" in the new
  process.  But in the child process nothing would ever release the
  lock, since the other threads weren't replicated, and the child
  process would no longer be able to perform imports.

  Python 2.7 acquires the import lock before performing an
  "os.fork()", and will also clean up any locks created using the
  "threading" module.  C extension modules that have internal locks,
  or that call "fork()" themselves, will not benefit from this clean-
  up.

  (Fixed by Thomas Wouters; bpo-1590864.)

* The "Py_Finalize()" function now calls the internal
  "threading._shutdown()" function; this prevents some exceptions from
  being raised when an interpreter shuts down. (Patch by Adam Olsen;
  bpo-1722344.)

* When using the "PyMemberDef" structure to define attributes of a
  type, Python will no longer let you try to delete or set a
  "T_STRING_INPLACE" attribute.

* Global symbols defined by the "ctypes" module are now prefixed with
  "Py", or with "_ctypes".  (Implemented by Thomas Heller; bpo-3102.)

* New configure option: the "--with-system-expat" switch allows
  building the "pyexpat" module to use the system Expat library.
  (Contributed by Arfrever Frehtes Taifersar Arahesis; bpo-7609.)

* New configure option: the "--with-valgrind" option will now disable
  the pymalloc allocator, which is difficult for the Valgrind memory-
  error detector to analyze correctly. Valgrind will therefore be
  better at detecting memory leaks and overruns. (Contributed by James
  Henstridge; bpo-2422.)

* New configure option: you can now supply an empty string to "--with-
  dbmliborder=" in order to disable all of the various DBM modules.
  (Added by Arfrever Frehtes Taifersar Arahesis; bpo-6491.)

* The **configure** script now checks for floating-point rounding bugs
  on certain 32-bit Intel chips and defines a "X87_DOUBLE_ROUNDING"
  preprocessor definition.  No code currently uses this definition,
  but it's available if anyone wishes to use it. (Added by Mark
  Dickinson; bpo-2937.)

  **configure** also now sets a "LDCXXSHARED" Makefile variable for
  supporting C++ linking.  (Contributed by Arfrever Frehtes Taifersar
  Arahesis; bpo-1222585.)

* The build process now creates the necessary files for pkg-config
  support.  (Contributed by Clinton Roy; bpo-3585.)

* The build process now supports Subversion 1.7.  (Contributed by
  Arfrever Frehtes Taifersar Arahesis; bpo-6094.)


Capsules
--------

Python 3.1 adds a new C datatype, "PyCapsule", for providing a C API
to an extension module.  A capsule is essentially the holder of a C
"void *" pointer, and is made available as a module attribute; for
example, the "socket" module's API is exposed as "socket.CAPI", and
"unicodedata" exposes "ucnhash_CAPI".  Other extensions can import the
module, access its dictionary to get the capsule object, and then get
the "void *" pointer, which will usually point to an array of pointers
to the module's various API functions.

There is an existing data type already used for this, "PyCObject", but
it doesn't provide type safety.  Evil code written in pure Python
could cause a segmentation fault by taking a "PyCObject" from module A
and somehow substituting it for the "PyCObject" in module B.
Capsules know their own name, and getting the pointer requires
providing the name:

   void *vtable;

   if (!PyCapsule_IsValid(capsule, "mymodule.CAPI") {
           PyErr_SetString(PyExc_ValueError, "argument type invalid");
           return NULL;
   }

   vtable = PyCapsule_GetPointer(capsule, "mymodule.CAPI");

You are assured that "vtable" points to whatever you're expecting. If
a different capsule was passed in, "PyCapsule_IsValid()" would detect
the mismatched name and return false.  Refer to Proporcionar una API C
para un módulo de extensión for more information on using these
objects.

Python 2.7 now uses capsules internally to provide various extension-
module APIs, but the "PyCObject_AsVoidPtr()" was modified to handle
capsules, preserving compile-time compatibility with the "CObject"
interface.  Use of "PyCObject_AsVoidPtr()" will signal a
"PendingDeprecationWarning", which is silent by default.

Implemented in Python 3.1 and backported to 2.7 by Larry Hastings;
discussed in bpo-5630.


Port-Specific Changes: Windows
------------------------------

* The "msvcrt" module now contains some constants from the
  "crtassem.h" header file: "CRT_ASSEMBLY_VERSION",
  "VC_ASSEMBLY_PUBLICKEYTOKEN", and "LIBRARIES_ASSEMBLY_NAME_PREFIX".
  (Contributed by David Cournapeau; bpo-4365.)

* The "_winreg" module for accessing the registry now implements the
  "CreateKeyEx()" and "DeleteKeyEx()" functions, extended versions of
  previously-supported functions that take several extra arguments.
  The "DisableReflectionKey()", "EnableReflectionKey()", and
  "QueryReflectionKey()" were also tested and documented. (Implemented
  by Brian Curtin: bpo-7347.)

* The new "_beginthreadex()" API is used to start threads, and the
  native thread-local storage functions are now used. (Contributed by
  Kristján Valur Jónsson; bpo-3582.)

* The "os.kill()" function now works on Windows.  The signal value can
  be the constants "CTRL_C_EVENT", "CTRL_BREAK_EVENT", or any integer.
  The first two constants will send "Control-C" and "Control-Break"
  keystroke events to subprocesses; any other value will use the
  "TerminateProcess()" API.  (Contributed by Miki Tebeka;
  bpo-1220212.)

* The "os.listdir()" function now correctly fails for an empty path.
  (Fixed by Hirokazu Yamamoto; bpo-5913.)

* The "mimelib" module will now read the MIME database from the
  Windows registry when initializing. (Patch by Gabriel Genellina;
  bpo-4969.)


Port-Specific Changes: Mac OS X
-------------------------------

* The path "/Library/Python/2.7/site-packages" is now appended to
  "sys.path", in order to share added packages between the system
  installation and a user-installed copy of the same version. (Changed
  by Ronald Oussoren; bpo-4865.)

     Distinto en la versión 2.7.13: As of 2.7.13, this change was
     removed. "/Library/Python/2.7/site-packages", the site-packages
     directory used by the Apple-supplied system Python 2.7 is no
     longer appended to "sys.path" for user-installed Pythons such as
     from the python.org installers.  As of macOS 10.12, Apple changed
     how the system site-packages directory is configured, which could
     cause installation of pip components, like setuptools, to fail.
     Packages installed for the system Python will no longer be shared
     with user-installed Pythons. (bpo-28440)


Port-Specific Changes: FreeBSD
------------------------------

* FreeBSD 7.1's "SO_SETFIB" constant, used with
  "getsockopt()"/"setsockopt()" to select an alternate routing table,
  is now available in the "socket" module.  (Added by Kyle VanderBeek;
  bpo-8235.)


Other Changes and Fixes
=======================

* Two benchmark scripts, "iobench" and "ccbench", were added to the
  "Tools" directory.  "iobench" measures the speed of the built-in
  file I/O objects returned by "open()" while performing various
  operations, and "ccbench" is a concurrency benchmark that tries to
  measure computing throughput, thread switching latency, and IO
  processing bandwidth when performing several tasks using a varying
  number of threads.

* The "Tools/i18n/msgfmt.py" script now understands plural forms in
  ".po" files.  (Fixed by Martin von Löwis; bpo-5464.)

* When importing a module from a ".pyc" or ".pyo" file with an
  existing ".py" counterpart, the "co_filename" attributes of the
  resulting code objects are overwritten when the original filename is
  obsolete.  This can happen if the file has been renamed, moved, or
  is accessed through different paths.  (Patch by Ziga Seilnacht and
  Jean-Paul Calderone; bpo-1180193.)

* The "regrtest.py" script now takes a "--randseed=" switch that takes
  an integer that will be used as the random seed for the "-r" option
  that executes tests in random order. The "-r" option also reports
  the seed that was used (Added by Collin Winter.)

* Another "regrtest.py" switch is "-j", which takes an integer
  specifying how many tests run in parallel. This allows reducing the
  total runtime on multi-core machines. This option is compatible with
  several other options, including the "-R" switch which is known to
  produce long runtimes. (Added by Antoine Pitrou, bpo-6152.)  This
  can also be used with a new "-F" switch that runs selected tests in
  a loop until they fail.  (Added by Antoine Pitrou; bpo-7312.)

* When executed as a script, the "py_compile.py" module now accepts
  "'-'" as an argument, which will read standard input for the list of
  filenames to be compiled.  (Contributed by Piotr Ożarowski;
  bpo-8233.)


Porting to Python 2.7
=====================

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

* The "range()" function processes its arguments more consistently; it
  will now call "__int__()" on non-float, non-integer arguments that
  are supplied to it.  (Fixed by Alexander Belopolsky; bpo-1533.)

* The string "format()" method changed the default precision used for
  floating-point and complex numbers from 6 decimal places to 12,
  which matches the precision used by "str()". (Changed by Eric Smith;
  bpo-5920.)

* Because of an optimization for the "with" statement, the special
  methods "__enter__()" and "__exit__()" must belong to the object's
  type, and cannot be directly attached to the object's instance.
  This affects new-style classes (derived from "object") and C
  extension types.  (bpo-6101.)

* Due to a bug in Python 2.6, the *exc_value* parameter to
  "__exit__()" methods was often the string representation of the
  exception, not an instance.  This was fixed in 2.7, so *exc_value*
  will be an instance as expected.  (Fixed by Florent Xicluna;
  bpo-7853.)

* Cuando se estableció un conjunto restringido de atributos mediante
  "__slots__", eliminar un atributo desarmado no llamaría
  "AttributeError" como cabría esperar.  Corregido por Benjamin
  Peterson; bpo-7604.)

In the standard library:

* Operations with "datetime" instances that resulted in a year falling
  outside the supported range didn't always raise "OverflowError".
  Such errors are now checked more carefully and will now raise the
  exception. (Reported by Mark Leander, patch by Anand B. Pillai and
  Alexander Belopolsky; bpo-7150.)

* When using "Decimal" instances with a string's "format()" method,
  the default alignment was previously left-alignment.  This has been
  changed to right-alignment, which might change the output of your
  programs. (Changed by Mark Dickinson; bpo-6857.)

  Comparisons involving a signaling NaN value (or "sNAN") now signal
  "InvalidOperation" instead of silently returning a true or false
  value depending on the comparison operator.  Quiet NaN values (or
  "NaN") are now hashable.  (Fixed by Mark Dickinson; bpo-7279.)

* The ElementTree library, "xml.etree", no longer escapes ampersands
  and angle brackets when outputting an XML processing instruction
  (which looks like *<?xml-stylesheet href="#style1"?>*) or comment
  (which looks like *<!-- comment -->*). (Patch by Neil Muller;
  bpo-2746.)

* The "readline()" method of "StringIO" objects now does nothing when
  a negative length is requested, as other file-like objects do.
  (bpo-7348).

* The "syslog" module will now use the value of "sys.argv[0]" as the
  identifier instead of the previous default value of "'python'".
  (Changed by Sean Reifschneider; bpo-8451.)

* The "tarfile" module's default error handling has changed, to no
  longer suppress fatal errors.  The default error level was
  previously 0, which meant that errors would only result in a message
  being written to the debug log, but because the debug log is not
  activated by default, these errors go unnoticed.  The default error
  level is now 1, which raises an exception if there's an error.
  (Changed by Lars Gustäbel; bpo-7357.)

* The "urlparse" module's "urlsplit()" now handles unknown URL schemes
  in a fashion compliant with **RFC 3986**: if the URL is of the form
  ""<something>://..."", the text before the "://" is treated as the
  scheme, even if it's a made-up scheme that the module doesn't know
  about.  This change may break code that worked around the old
  behaviour.  For example, Python 2.6.4 or 2.5 will return the
  following:

     >>> import urlparse
     >>> urlparse.urlsplit('invented://host/filename?query')
     ('invented', '', '//host/filename?query', '', '')

  Python 2.7 (and Python 2.6.5) will return:

     >>> import urlparse
     >>> urlparse.urlsplit('invented://host/filename?query')
     ('invented', 'host', '/filename?query', '', '')

  (Python 2.7 actually produces slightly different output, since it
  returns a named tuple instead of a standard tuple.)

For C extensions:

* C extensions that use integer format codes with the "PyArg_Parse*"
  family of functions will now raise a "TypeError" exception instead
  of triggering a "DeprecationWarning" (bpo-5080).

* Use the new "PyOS_string_to_double()" function instead of the old
  "PyOS_ascii_strtod()" and "PyOS_ascii_atof()" functions, which are
  now deprecated.

For applications that embed Python:

* The "PySys_SetArgvEx()" function was added, letting applications
  close a security hole when the existing "PySys_SetArgv()" function
  was used.  Check whether you're calling "PySys_SetArgv()" and
  carefully consider whether the application should be using
  "PySys_SetArgvEx()" with *updatepath* set to false.


New Features Added to Python 2.7 Maintenance Releases
=====================================================

New features may be added to Python 2.7 maintenance releases when the
situation genuinely calls for it. Any such additions must go through
the Python Enhancement Proposal process, and make a compelling case
for why they can't be adequately addressed by either adding the new
feature solely to Python 3, or else by publishing it on the Python
Package Index.

In addition to the specific proposals listed below, there is a general
exemption allowing new "-3" warnings to be added in any Python 2.7
maintenance release.


Two new environment variables for debug mode
--------------------------------------------

In debug mode, the "[xxx refs]" statistic is not written by default,
the "PYTHONSHOWREFCOUNT" environment variable now must also be set.
(Contributed by Victor Stinner; bpo-31733.)

When Python is compiled with "COUNT_ALLOC" defined, allocation counts
are no longer dumped by default anymore: the "PYTHONSHOWALLOCCOUNT"
environment variable must now also be set. Moreover, allocation counts
are now dumped into stderr, rather than stdout. (Contributed by Victor
Stinner; bpo-31692.)

Nuevo en la versión 2.7.15.


PEP 434: IDLE Enhancement Exception for All Branches
----------------------------------------------------

**PEP 434** describes a general exemption for changes made to the IDLE
development environment shipped along with Python. This exemption
makes it possible for the IDLE developers to provide a more consistent
user experience across all supported versions of Python 2 and 3.

For details of any IDLE changes, refer to the NEWS file for the
specific release.


PEP 466: Network Security Enhancements for Python 2.7
-----------------------------------------------------

**PEP 466** describes a number of network security enhancement
proposals that have been approved for inclusion in Python 2.7
maintenance releases, with the first of those changes appearing in the
Python 2.7.7 release.

**PEP 466** related features added in Python 2.7.7:

* "hmac.compare_digest()" was backported from Python 3 to make a
  timing attack resistant comparison operation available to Python 2
  applications. (Contributed by Alex Gaynor; bpo-21306.)

* OpenSSL 1.0.1g was upgraded in the official Windows installers
  published on python.org. (Contributed by Zachary Ware; bpo-21462.)

**PEP 466** related features added in Python 2.7.8:

* "hashlib.pbkdf2_hmac()" was backported from Python 3 to make a
  hashing algorithm suitable for secure password storage broadly
  available to Python 2 applications. (Contributed by Alex Gaynor;
  bpo-21304.)

* OpenSSL 1.0.1h was upgraded for the official Windows installers
  published on python.org. (contributed by Zachary Ware in bpo-21671
  for CVE-2014-0224)

**PEP 466** related features added in Python 2.7.9:

* Most of Python 3.4's "ssl" module was backported. This means "ssl"
  now supports Server Name Indication, TLS1.x settings, access to the
  platform certificate store, the "SSLContext" class, and other
  features. (Contributed by Alex Gaynor and David Reid; bpo-21308.)

  Refer to the "Version added: 2.7.9" notes in the module
  documentation for specific details.

* "os.urandom()" was changed to cache a file descriptor to
  "/dev/urandom" instead of reopening "/dev/urandom" on every call.
  (Contributed by Alex Gaynor; bpo-21305.)

* "hashlib.algorithms_guaranteed" and "hashlib.algorithms_available"
  were backported from Python 3 to make it easier for Python 2
  applications to select the strongest available hash algorithm.
  (Contributed by Alex Gaynor in bpo-21307)


PEP 477: Backport ensurepip (PEP 453) to Python 2.7
---------------------------------------------------

**PEP 477** approves the inclusion of the **PEP 453** ensurepip module
and the improved documentation that was enabled by it in the Python
2.7 maintenance releases, appearing first in the Python 2.7.9 release.


Bootstrapping pip By Default
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The new "ensurepip" module (defined in **PEP 453**) provides a
standard cross-platform mechanism to bootstrap the pip installer into
Python installations. The version of "pip" included with Python 2.7.9
is "pip" 1.5.6, and future 2.7.x maintenance releases will update the
bundled version to the latest version of "pip" that is available at
the time of creating the release candidate.

By default, the commands "pip", "pipX" and "pipX.Y" will be installed
on all platforms (where X.Y stands for the version of the Python
installation), along with the "pip" Python package and its
dependencies.

For CPython source builds on POSIX systems, the "make install" and
"make altinstall" commands do not bootstrap "pip" by default.  This
behaviour can be controlled through configure options, and overridden
through Makefile options.

On Windows and Mac OS X, the CPython installers now default to
installing "pip" along with CPython itself (users may opt out of
installing it during the installation process). Window users will need
to opt in to the automatic "PATH" modifications to have "pip"
available from the command line by default, otherwise it can still be
accessed through the Python launcher for Windows as "py -m pip".

As discussed in the PEP, platform packagers may choose not to install
these commands by default, as long as, when invoked, they provide
clear and simple directions on how to install them on that platform
(usually using the system package manager).


Documentation Changes
~~~~~~~~~~~~~~~~~~~~~

As part of this change, the Instalando módulos de Python and
Distribuir módulos de Python sections of the documentation have been
completely redesigned as short getting started and FAQ documents. Most
packaging documentation has now been moved out to the Python Packaging
Authority maintained Python Packaging User Guide and the documentation
of the individual projects.

However, as this migration is currently still incomplete, the legacy
versions of those guides remaining available as Instalación de Módulos
de Python (versión antigua) and Distribución de módulos de Python
(versión heredada).

Ver también:

  **PEP 453** -- Explicit bootstrapping of pip in Python installations
     PEP written by Donald Stufft and Nick Coghlan, implemented by
     Donald Stufft, Nick Coghlan, Martin von Löwis and Ned Deily.


PEP 476: Enabling certificate verification by default for stdlib http clients
-----------------------------------------------------------------------------

**PEP 476** updated "httplib" and modules which use it, such as
"urllib2" and "xmlrpclib", to now verify that the server presents a
certificate which is signed by a Certificate Authority in the platform
trust store and whose hostname matches the hostname being requested by
default, significantly improving security for many applications. This
change was made in the Python 2.7.9 release.

For applications which require the old previous behavior, they can
pass an alternate context:

   import urllib2
   import ssl

   # This disables all verification
   context = ssl._create_unverified_context()

   # This allows using a specific certificate for the host, which doesn't need
   # to be in the trust store
   context = ssl.create_default_context(cafile="/path/to/file.crt")

   urllib2.urlopen("https://invalid-cert", context=context)


PEP 493: HTTPS verification migration tools for Python 2.7
----------------------------------------------------------

**PEP 493** provides additional migration tools to support a more
incremental infrastructure upgrade process for environments containing
applications and services relying on the historically permissive
processing of server certificates when establishing client HTTPS
connections.  These additions were made in the Python 2.7.12 release.

These tools are intended for use in cases where affected applications
and services can't be modified to explicitly pass a more permissive
SSL context when establishing the connection.

For applications and services which can't be modified at all, the new
"PYTHONHTTPSVERIFY" environment variable may be set to "0" to revert
an entire Python process back to the default permissive behaviour of
Python 2.7.8 and earlier.

For cases where the connection establishment code can't be modified,
but the overall application can be, the new
"ssl._https_verify_certificates()" function can be used to adjust the
default behaviour at runtime.


New "make regen-all" build target
---------------------------------

To simplify cross-compilation, and to ensure that CPython can reliably
be compiled without requiring an existing version of Python to already
be available, the autotools-based build system no longer attempts to
implicitly recompile generated files based on file modification times.

Instead, a new "make regen-all" command has been added to force
regeneration of these files when desired (e.g. after an initial
version of Python has already been built based on the pregenerated
versions).

More selective regeneration targets are also defined - see
Makefile.pre.in for details.

(Contributed by Victor Stinner in bpo-23404.)

Nuevo en la versión 2.7.14.


Removal of "make touch" build target
------------------------------------

The "make touch" build target previously used to request implicit
regeneration of generated files by updating their modification times
has been removed.

It has been replaced by the new "make regen-all" target.

(Contributed by Victor Stinner in bpo-23404.)

Distinto en la versión 2.7.14.


Acknowledgements
================

The author would like to thank the following people for offering
suggestions, corrections and assistance with various drafts of this
article: Nick Coghlan, Philip Jenvey, Ryan Lovett, R. David Murray,
Hugh Secker-Walker.
