cpython/Doc/library/gdbm.rst

:mod:`gdbm` --- GNU's reinterpretation of dbm
=============================================

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


.. index:: module: dbm

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

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

The module defines the following constant and functions:


.. exception:: error

   Raised on ``gdbm``\ -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 ``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 ``0666``.

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


.. function:: 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.


.. function:: 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)


.. function:: 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``
   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.


.. function:: sync()

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


.. seealso::

   Module :mod:`anydbm`
      Generic interface to ``dbm``\ -style databases.

   Module :mod:`whichdb`
      Utility module used to determine the type of an existing database.