dbm — Interfaces para «bases de datos» de Unix

Código fuente: Lib/dbm/__init__.py

dbm es una interfaz genérica para variantes de la base de datos DBM — dbm.gnu o dbm.ndbm. Si ninguno de estos módulos son instalados, se utilizará la implementación lenta pero sencilla en el módulo dbm.dumb. Existe una interfaz de terceros para la Oracle Berkeley DB.

exception dbm.error

Una tupla que contiene las excepciones que pueden ser lanzadas por cada uno de los módulos soportados, con una excepción única también denominada dbm.error como el primer elemento — el último se usa cuando se genera dbm.error.

dbm.whichdb(filename)

Esta función intenta adivinar cuál de los varios módulos de base de datos simples disponibles — dbm.gnu, dbm.ndbm o dbm.dumb — deberán usarse para abrir un archivo.

Return one of the following values:

  • None if the file can’t be opened because it’s unreadable or doesn’t exist

  • the empty string ('') if the file’s format can’t be guessed

  • a string containing the required module name, such as 'dbm.ndbm' or 'dbm.gnu'

Distinto en la versión 3.11: filename accepts a path-like object.

dbm.open(file, flag='r', mode=0o666)

Open a database and return the corresponding database object.

Parámetros:

  • file (path-like object) –

    The database file to open.

    If the database file already exists, the whichdb() function is used to determine its type and the appropriate module is used; if it does not exist, the first submodule listed above that can be imported is used.

  • flag (str) –

    • 'r' (default): Open existing database for reading only.

    • 'w': Open existing database for reading and writing.

    • 'c': Open database for reading and writing, creating it if it doesn’t exist.

    • 'n': Always create a new, empty database, open for reading and writing.

  • mode (int) – The Unix file access mode of the file (default: octal 0o666), used only when the database has to be created.

Distinto en la versión 3.11: file accepts a path-like object.

The object returned by open() supports the same basic functionality as a dict; keys and their corresponding values can be stored, retrieved, and deleted, and the in operator and the keys() method are available, as well as get() and setdefault() methods.

Key and values are always stored as bytes. This means that when strings are used they are implicitly converted to the default encoding before being stored.

Estos objetos también admiten el uso en una instrucción with, que los cerrará automáticamente cuando termine.

Distinto en la versión 3.2: get() and setdefault() methods are now available for all dbm backends.

Distinto en la versión 3.4: Added native support for the context management protocol to the objects returned by open().

Distinto en la versión 3.8: Deleting a key from a read-only database raises a database module specific exception instead of KeyError.

El siguiente ejemplo registra algunos nombres de host y un título correspondiente, y luego imprime el contenido de la base de datos:

import dbm

# Open database, creating it if necessary.
with dbm.open('cache', 'c') as db:

    # Record some values
    db[b'hello'] = b'there'
    db['www.python.org'] = 'Python Website'
    db['www.cnn.com'] = 'Cable News Network'

    # Note that the keys are considered bytes now.
    assert db[b'www.python.org'] == b'Python Website'
    # Notice how the value is now in bytes.
    assert db['www.cnn.com'] == b'Cable News Network'

    # Often-used methods of the dict interface work too.
    print(db.get('python.org', b'not present'))

    # Storing a non-string key or value will raise an exception (most
    # likely a TypeError).
    db['www.yahoo.com'] = 4

# db is automatically closed when leaving the with statement.

Ver también

Módulo shelve

Módulo de persistencia que almacena datos que no son cadenas de caracteres.

Los submódulos individuales se describen en las siguientes secciones.

dbm.gnu — GNU database manager

Código fuente: Lib/dbm/gnu.py

The dbm.gnu module provides an interface to the GDBM library, similar to the dbm.ndbm module, but with additional functionality like crash tolerance.

Nota

The file formats created by dbm.gnu and dbm.ndbm are incompatible and can not be used interchangeably.

exception dbm.gnu.error

Se lanza en errores específicos dbm.gnu, como errores de E/S. KeyError se genera para errores generales de asignación, como especificar una clave incorrecta.

dbm.gnu.open(filename, flag='r', mode=0o666, /)

Open a GDBM database and return a gdbm object.

Parámetros:

  • filename (path-like object) – The database file to open.

  • flag (str) –

    • 'r' (default): Open existing database for reading only.

    • 'w': Open existing database for reading and writing.

    • 'c': Open database for reading and writing, creating it if it doesn’t exist.

    • 'n': Always create a new, empty database, open for reading and writing.

    The following additional characters may be appended to control how the database is opened:

    • 'f': Open the database in fast mode. Writes to the database will not be synchronized.

    • 's': Synchronized mode. Changes to the database will be written immediately to the file.

    • 'u': Do not lock database.

    Not all flags are valid for all versions of GDBM. See the open_flags member for a list of supported flag characters.

  • mode (int) – The Unix file access mode of the file (default: octal 0o666), used only when the database has to be created.

Muestra:

error – If an invalid flag argument is passed.

Distinto en la versión 3.11: filename accepts a path-like object.

dbm.gnu.open_flags

A string of characters the flag parameter of open() supports.

gdbm objects behave similar to mappings, but items() and values() methods are not supported. The following methods are also provided:

gdbm.firstkey()

It’s possible to loop over every key in the database using this method and the nextkey() method. The traversal is ordered by GDBM’s internal hash values, and won’t be sorted by the key values. This method returns the starting key.

gdbm.nextkey(key)

Retorna la clave que sigue a key en el recorrido. El siguiente código imprime todas las claves en la base de datos db, sin tener que crear una lista en la memoria que las contenga todas:

k = db.firstkey()
while k is not None:
    print(k)
    k = db.nextkey(k)
gdbm.reorganize()

If you have carried out a lot of deletions and would like to shrink the space used by the GDBM file, this routine will reorganize the database. gdbm objects will not shorten the length of a database file except by using this reorganization; otherwise, deleted file space will be kept and reused as new (key, value) pairs are added.

gdbm.sync()

Cuando la base de datos se ha abierto en modo rápido, este método obliga a que los datos no escritos se escriban en el disco.

gdbm.close()

Close the GDBM database.

dbm.ndbm — New Database Manager

Código fuente: Lib/dbm/ndbm.py

The dbm.ndbm module provides an interface to the NDBM library. This module can be used with the «classic» NDBM interface or the GDBM compatibility interface.

Nota

The file formats created by dbm.gnu and dbm.ndbm are incompatible and can not be used interchangeably.

Advertencia

The NDBM library shipped as part of macOS has an undocumented limitation on the size of values, which can result in corrupted database files when storing values larger than this limit. Reading such corrupted files can result in a hard crash (segmentation fault).

exception dbm.ndbm.error

Se lanza en errores específicos bm.ndbm, como errores de E/S. KeyError lanza para errores generales de asignación, como especificar una clave incorrecta.

dbm.ndbm.library

Name of the NDBM implementation library used.

dbm.ndbm.open(filename, flag='r', mode=0o666, /)

Open an NDBM database and return an ndbm object.

Parámetros:

  • filename (path-like object) – The basename of the database file (without the .dir or .pag extensions).

  • flag (str) –

    • 'r' (default): Open existing database for reading only.

    • 'w': Open existing database for reading and writing.

    • 'c': Open database for reading and writing, creating it if it doesn’t exist.

    • 'n': Always create a new, empty database, open for reading and writing.

  • mode (int) – The Unix file access mode of the file (default: octal 0o666), used only when the database has to be created.

ndbm objects behave similar to mappings, but items() and values() methods are not supported. The following methods are also provided:

Distinto en la versión 3.11: Acepta path-like object como nombre de archivo.

ndbm.close()

Close the NDBM database.

dbm.dumb — Implementación de DBM portátil

Código fuente: Lib/dbm/dumb.py

Nota

El módulo dbm.dumb está pensado como último recurso para el módulo dbm cuando no hay disponible un módulo más robusto. El módulo dbm.dymb no está escrito para velocidad y no se usa tanto como los otros módulos de base de datos.

The dbm.dumb module provides a persistent dict-like interface which is written entirely in Python. Unlike other dbm backends, such as dbm.gnu, no external library is required.

The dbm.dumb module defines the following:

exception dbm.dumb.error

Se lanza en errores específicos dbm.dumb, como errores de E/S. KeyError se lanza para errores de mapeo generales como especificar una clave incorrecta.

dbm.dumb.open(filename, flag='c', mode=0o666)

Open a dbm.dumb database. The returned database object behaves similar to a mapping, in addition to providing sync() and close() methods.

Parámetros:

  • filename –

    The basename of the database file (without extensions). A new database creates the following files:

    • filename.dat

    • filename.dir

  • flag (str) –

    • 'r': Open existing database for reading only.

    • 'w': Open existing database for reading and writing.

    • 'c' (default): Open database for reading and writing, creating it if it doesn’t exist.

    • 'n': Always create a new, empty database, open for reading and writing.

  • mode (int) – The Unix file access mode of the file (default: octal 0o666), used only when the database has to be created.

Advertencia

Es posible bloquear el intérprete de Python cuando se carga una base de datos con una entrada suficientemente grande/compleja debido a las limitaciones de profundidad de la pila en el compilador AST de Python.

Distinto en la versión 3.5: open() always creates a new database when flag is 'n'.

Distinto en la versión 3.8: A database opened read-only if flag is 'r'. A database is not created if it does not exist if flag is 'r' or 'w'.

Distinto en la versión 3.11: filename accepts a path-like object.

In addition to the methods provided by the collections.abc.MutableMapping class, the following methods are provided:

dumbdbm.sync()

Sincroniza el directorio en disco y los archivos de datos. Este método es llamado por el método Shelve.sync().

dumbdbm.close()

Close the database.