:mod:`dbm` --- Interfaces to Unix "databases"

.. module:: dbm
 :synopsis: Interfaces to various Unix "database" formats.

Source code: :source:`Lib/dbm/__init__.py`

:mod:`dbm` is a generic interface to variants of the DBM database --- :mod:`dbm.gnu` or :mod:`dbm.ndbm`. If none of these modules is installed, the slow-but-simple implementation in module :mod:`dbm.dumb` will be used. There is a :func:`.open` supports the same basic functionality as dictionaries; keys and their corresponding values can be stored, retrieved, and deleted, and the :keyword:`in` operator and the :meth:`keys` method are available, as well as :meth:`get` and :meth:`setdefault`.

.. versionchanged:: 3.2
 :meth:`get` and :meth:`setdefault` are now available in all database modules.

.. versionchanged:: 3.8
 Deleting a key from a read-only database raises database module specific error
 instead of :exc:`KeyError`.

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.

These objects also support being used in a :keyword:`with` statement, which will automatically close them when done.

.. versionchanged:: 3.4
 Added native support for the context management protocol to the objects
 returned by :func:`.open`.

The following example records some hostnames and a corresponding title, and then prints out the contents of the database:

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.

.. seealso::

 Module :mod:`shelve`
 Persistence module which stores non-string data.

The individual submodules are described in the following sections.

:mod:`dbm.gnu` --- GNU's reinterpretation of dbm

.. module:: dbm.gnu
 :platform: Unix
 :synopsis: GNU's reinterpretation of dbm.

Source code: :source:`Lib/dbm/gnu.py`

This module is quite similar to the :mod:`dbm` module, but uses the GNU library gdbm instead to provide some additional functionality. Please note that the file formats created by :mod:`dbm.gnu` and :mod:`dbm.ndbm` are incompatible.

The :mod:`dbm.gnu` module provides an interface to the GNU DBM library. dbm.gnu.gdbm objects behave like mappings (dictionaries), except that keys and values are always converted to bytes before storing. Printing a gdbm object doesn't print the keys and values, and the :meth:`items` and :meth:`values` methods are not supported.

.. exception:: error

 Raised on :mod:`dbm.gnu`-specific errors, such as I/O errors. :exc:`KeyError` is
 raised for general mapping errors like specifying an incorrect key.

.. function:: open(filename[, flag[, mode]])

 Open a ``gdbm`` database and return a :class:`gdbm` object. The *filename*
 argument is the name of the database file.

 The optional *flag* argument can be:

 +---------+-------------------------------------------+
 | Value | Meaning |
 +=========+===========================================+
 | ``'r'`` | Open existing database for reading only |
 | | (default) |
 +---------+-------------------------------------------+
 | ``'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 the flag to control
 how the database is opened:

 +---------+--------------------------------------------+
 | Value | Meaning |
 +=========+============================================+
 | ``'f'`` | Open the database in fast mode. Writes |
 | | to the database will not be synchronized. |
 +---------+--------------------------------------------+
 | ``'s'`` | Synchronized mode. This will cause changes |
 | | to the database to be immediately written |
 | | to the file. |
 +---------+--------------------------------------------+
 | ``'u'`` | Do not lock database. |
 +---------+--------------------------------------------+

 Not all flags are valid for all versions of ``gdbm``. The module constant
 :const:`open_flags` is a string of supported flag characters. The exception
 :exc:`error` is raised if an invalid flag is specified.

 The optional *mode* argument is the Unix mode of the file, used only when the
 database has to be created. It defaults to octal ``0o666``.

 In addition to the dictionary-like methods, ``gdbm`` objects have the
 following methods:

 .. method:: gdbm.firstkey()

 It's possible to loop over every key in the database using this method and the
 :meth:`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.

 .. method:: gdbm.nextkey(key)

 Returns the key that follows *key* in the traversal. The following code prints
 every key in the database ``db``, without having to create a list in memory that
 contains them all::

 k = db.firstkey()
 while k != None:
 print(k)
 k = db.nextkey(k)

 .. method:: 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.

 .. method:: gdbm.sync()

 When the database has been opened in fast mode, this method forces any
 unwritten data to be written to the disk.

 .. method:: gdbm.close()

 Close the ``gdbm`` database.

:mod:`dbm.ndbm` --- Interface based on ndbm

.. module:: dbm.ndbm
 :platform: Unix
 :synopsis: The standard "database" interface, based on ndbm.

Source code: :source:`Lib/dbm/ndbm.py`

The :mod:`dbm.ndbm` module provides an interface to the Unix "(n)dbm" library. Dbm objects behave like mappings (dictionaries), except that keys and values are always stored as bytes. Printing a dbm object doesn't print the keys and values, and the :meth:`items` and :meth:`values` methods are not supported.

This module can be used with the "classic" ndbm interface or the GNU GDBM compatibility interface. On Unix, the :program:`configure` script will attempt to locate the appropriate header file to simplify building this module.

.. exception:: error

 Raised on :mod:`dbm.ndbm`-specific errors, such as I/O errors. :exc:`KeyError` is raised
 for general mapping errors like specifying an incorrect key.

.. data:: library

 Name of the ``ndbm`` implementation library used.

.. function:: open(filename[, flag[, mode]])

 Open a dbm database and return a ``ndbm`` object. The *filename* argument is the
 name of the database file (without the :file:`.dir` or :file:`.pag` extensions).

 The optional *flag* argument must be one of these values:

 +---------+-------------------------------------------+
 | Value | Meaning |
 +=========+===========================================+
 | ``'r'`` | Open existing database for reading only |
 | | (default) |
 +---------+-------------------------------------------+
 | ``'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 optional *mode* argument is the Unix mode of the file, used only when the
 database has to be created. It defaults to octal ``0o666`` (and will be
 modified by the prevailing umask).

 In addition to the dictionary-like methods, ``ndbm`` objects
 provide the following method:

 .. method:: ndbm.close()

 Close the ``ndbm`` database.

:mod:`dbm.dumb` --- Portable DBM implementation

.. module:: dbm.dumb
 :synopsis: Portable implementation of the simple DBM interface.

Source code: :source:`Lib/dbm/dumb.py`

.. index:: single: databases

The :mod:`dbm.dumb` module provides a persistent dictionary-like interface which is written entirely in Python. Unlike other modules such as :mod:`dbm.gnu` no external library is required. As with other persistent mappings, the keys and values are always stored as bytes.

The module defines the following:

.. exception:: error

 Raised on :mod:`dbm.dumb`-specific errors, such as I/O errors. :exc:`KeyError` is
 raised for general mapping errors like specifying an incorrect key.

.. function:: open(filename[, flag[, mode]])

 Open a ``dumbdbm`` database and return a dumbdbm object. The *filename* argument is
 the basename of the database file (without any specific extensions). When a
 dumbdbm database is created, files with :file:`.dat` and :file:`.dir` extensions
 are created.

 The optional *flag* argument can be:

 +---------+-------------------------------------------+
 | Value | Meaning |
 +=========+===========================================+
 | ``'r'`` | Open existing database for reading only |
 | | (default) |
 +---------+-------------------------------------------+
 | ``'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 optional *mode* argument is the Unix mode of the file, used only when the
 database has to be created. It defaults to octal ``0o666`` (and will be modified
 by the prevailing umask).

 .. warning::
 It is possible to crash the Python interpreter when loading a database
 with a sufficiently large/complex entry due to stack depth limitations in
 Python's AST compiler.

 .. versionchanged:: 3.5
 :func:`.open` always creates a new database when the flag has the value
 ``'n'``.

 .. versionchanged:: 3.8
 A database opened with flags ``'r'`` is now read-only. Opening with
 flags ``'r'`` and ``'w'`` no longer creates a database if it does not
 exist.

 In addition to the methods provided by the
 :class:`collections.abc.MutableMapping` class, :class:`dumbdbm` objects
 provide the following methods:

 .. method:: dumbdbm.sync()

 Synchronize the on-disk directory and data files. This method is called
 by the :meth:`Shelve.sync` method.

 .. method:: dumbdbm.close()

 Close the ``dumbdbm`` database.

此处可能存在不合适展示的内容,页面不予展示。您可通过相关编辑功能自查并修改。

如您确认内容无涉及 不当用语 / 纯广告导流 / 暴力 / 低俗色情 / 侵权 / 盗版 / 虚假 / 无价值内容或违法国家有关法律法规的内容,可点击提交进行申诉,我们将尽快为您处理。