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1:mod:`gdbm` --- GNU's reinterpretation of dbm
2=============================================
3
4.. module:: gdbm
5   :platform: Unix
6   :synopsis: GNU's reinterpretation of dbm.
7
8.. note::
9   The :mod:`gdbm` module has been renamed to :mod:`dbm.gnu` in Python 3.  The
10   :term:`2to3` tool will automatically adapt imports when converting your
11   sources to Python 3.
12
13
14.. index:: module: dbm
15
16This module is quite similar to the :mod:`dbm` module, but uses ``gdbm`` instead
17to provide some additional functionality.  Please note that the file formats
18created by ``gdbm`` and ``dbm`` are incompatible.
19
20The :mod:`gdbm` module provides an interface to the GNU DBM library.  ``gdbm``
21objects behave like mappings (dictionaries), except that keys and values are
22always strings. Printing a ``gdbm`` object doesn't print the keys and values,
23and the :meth:`items` and :meth:`values` methods are not supported.
24
25The module defines the following constant and functions:
26
27
28.. exception:: error
29
30   Raised on ``gdbm``\ -specific errors, such as I/O errors. :exc:`KeyError` is
31   raised for general mapping errors like specifying an incorrect key.
32
33
34.. function:: open(filename, [flag, [mode]])
35
36   Open a ``gdbm`` database and return a ``gdbm`` object.  The *filename* argument
37   is the name of the database file.
38
39   The optional *flag* argument can be:
40
41   +---------+-------------------------------------------+
42   | Value   | Meaning                                   |
43   +=========+===========================================+
44   | ``'r'`` | Open existing database for reading only   |
45   |         | (default)                                 |
46   +---------+-------------------------------------------+
47   | ``'w'`` | Open existing database for reading and    |
48   |         | writing                                   |
49   +---------+-------------------------------------------+
50   | ``'c'`` | Open database for reading and writing,    |
51   |         | creating it if it doesn't exist           |
52   +---------+-------------------------------------------+
53   | ``'n'`` | Always create a new, empty database, open |
54   |         | for reading and writing                   |
55   +---------+-------------------------------------------+
56
57   The following additional characters may be appended to the flag to control
58   how the database is opened:
59
60   +---------+--------------------------------------------+
61   | Value   | Meaning                                    |
62   +=========+============================================+
63   | ``'f'`` | Open the database in fast mode.  Writes    |
64   |         | to the database will not be synchronized.  |
65   +---------+--------------------------------------------+
66   | ``'s'`` | Synchronized mode. This will cause changes |
67   |         | to the database to be immediately written  |
68   |         | to the file.                               |
69   +---------+--------------------------------------------+
70   | ``'u'`` | Do not lock database.                      |
71   +---------+--------------------------------------------+
72
73   Not all flags are valid for all versions of ``gdbm``.  The module constant
74   :const:`open_flags` is a string of supported flag characters.  The exception
75   :exc:`error` is raised if an invalid flag is specified.
76
77   The optional *mode* argument is the Unix mode of the file, used only when the
78   database has to be created.  It defaults to octal ``0666``.
79
80In addition to the dictionary-like methods, ``gdbm`` objects have the following
81methods:
82
83
84.. function:: firstkey()
85
86   It's possible to loop over every key in the database using this method  and the
87   :meth:`nextkey` method.  The traversal is ordered by ``gdbm``'s internal hash
88   values, and won't be sorted by the key values.  This method returns the starting
89   key.
90
91
92.. function:: nextkey(key)
93
94   Returns the key that follows *key* in the traversal.  The following code prints
95   every key in the database ``db``, without having to create a list in memory that
96   contains them all::
97
98      k = db.firstkey()
99      while k != None:
100          print k
101          k = db.nextkey(k)
102
103
104.. function:: reorganize()
105
106   If you have carried out a lot of deletions and would like to shrink the space
107   used by the ``gdbm`` file, this routine will reorganize the database.  ``gdbm``
108   will not shorten the length of a database file except by using this
109   reorganization; otherwise, deleted file space will be kept and reused as new
110   (key, value) pairs are added.
111
112
113.. function:: sync()
114
115   When the database has been opened in fast mode, this method forces any
116   unwritten data to be written to the disk.
117
118
119.. function:: close()
120
121   Close the ``gdbm`` database.
122
123
124.. seealso::
125
126   Module :mod:`anydbm`
127      Generic interface to ``dbm``\ -style databases.
128
129   Module :mod:`whichdb`
130      Utility module used to determine the type of an existing database.
131
132