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Doc/library/dbm.rst
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Serhiy Storchaka
[3.9] bpo-45060: Get rid of few uses of the equality operators with None (GH-28087). (GH-28093)
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: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 `third party interface <https://www.jcea.es/programacion/pybsddb.htm>`_ to
the Oracle Berkeley DB.


.. exception:: error

   A tuple containing the exceptions that can be raised by each of the supported
   modules, with a unique exception also named :exc:`dbm.error` as the first
   item --- the latter is used when :exc:`dbm.error` is raised.


.. function:: whichdb(filename)

   This function attempts to guess which of the several simple database modules
   available --- :mod:`dbm.gnu`, :mod:`dbm.ndbm` or :mod:`dbm.dumb` --- should
   be used to open a given file.

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


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

   Open the database file *file* and return a corresponding object.

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

   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).


The object returned by :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 is not 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

.. note::

   The :mod:`dbm.dumb` module is intended as a last resort fallback for the
   :mod:`dbm` module when a more robust module is not available. The :mod:`dbm.dumb`
   module is not written for speed and is not nearly as heavily used as the other
   database modules.

--------------

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.