# `dbm` — Interfaces to Unix «databases»

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.