
``dbm`` --- Interfaces to Unix "databases"
******************************************

``dbm`` is a generic interface to variants of the DBM database ---
   ``dbm.gnu`` or ``dbm.ndbm``.  If none of these modules is
   installed, the slow-but-simple implementation in module
   ``dbm.dumb`` will be used.  There is a third party interface to the
   Oracle Berkely DB.

exception exception dbm.error

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

dbm.whichdb(filename)

   This functionattempts to guess which of the several simple database
   modules available --- ``dbm.bsd``, ``dbm.gnu``, ``dbm.ndbm`` or
   ``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'``.

dbm.open(filename[, flag[, mode]])

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

   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 module listed above that can be
   imported is used.

   The optional *flag* argument can be ``'r'`` to open an existing
   database for reading only, ``'w'`` to open an existing database for
   reading and writing, ``'c'`` to create the database if it doesn't
   exist, or ``'n'``, which will always create a new empty database.
   If not specified, the default value is ``'r'``.

   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 ``open()`` supports most of the same
functionality as dictionaries; keys and their corresponding values can
be stored, retrieved, and deleted, and the ``in`` operator and the
``keys()`` method are available. 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.

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.
   db = dbm.open('cache', 'c')

   # 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'

   # Loop through contents.  Other dictionary methods
   # such as .keys(), .values() also work.
   for k, v in db.iteritems():
       print(k, '\t', v)

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

   # Close when done.
   db.close()

See also:

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

The individual submodules are described in the following sections.


``dbm.gnu`` --- GNU's reinterpretation of dbm
=============================================

Platform: Unix

*Platforms: *Unix

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

The ``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
``items()`` and ``values()`` methods are not supported.

exception exception dbm.gnu.error

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

dbm.gnu.open(filename[, flag[, mode]])

   Open a ``gdbm`` database and return a ``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 ``open_flags`` is a string of supported flag characters.
   The exception ``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:

   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)

      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)

   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()

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


``dbm.ndbm`` --- Interface based on ndbm
========================================

Platform: Unix

*Platforms: *Unix

The ``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 ``items()`` and
``values()`` methods are not supported.

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

exception exception dbm.ndbm.error

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

dbm.ndbm.library

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

dbm.ndbm.open(filename[, flag[, mode]])

   Open a dbm database and return a ``dbm`` object.  The *filename*
   argument is the name of the database file (without the ``.dir`` or
   ``.pag`` extensions; note that the BSD DB implementation of the
   interface will append the extension ``.db`` and only create one
   file).

   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).


``dbm.dumb`` --- Portable DBM implementation
============================================

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

The ``dbm.dumb`` module provides a persistent dictionary-like
interface which is written entirely in Python.  Unlike other modules
such as ``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 exception dbm.dumb.error

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

dbm.dumb.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 ``.dat`` and ``.dir`` extensions are created.

   The optional *flag* argument is currently ignored; the database is
   always opened for update, and will be created if it does not exist.

   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 methods provided by the
   ``collections.MutableMapping`` class, ``dumbdbm`` objects provide
   the following method:

   dumbdbm.sync()

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