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:mod:`gdbm` --- GNU's reinterpretation of dbm
=============================================
.. module:: gdbm
:platform: Unix
:synopsis: GNU's reinterpretation of dbm.
.. note::
The :mod:`gdbm` module has been renamed to :mod:`dbm.gnu` in Python 3.0. The
:term:`2to3` tool will automatically adapt imports when converting your
sources to 3.0.
.. 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.
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