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ReStructuredText
122 lines
4.5 KiB
ReStructuredText
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:mod:`gdbm` --- GNU's reinterpretation of dbm
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=============================================
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.. module:: gdbm
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:platform: Unix
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:synopsis: GNU's reinterpretation of dbm.
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.. index:: module: dbm
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This module is quite similar to the :mod:`dbm` module, but uses ``gdbm`` instead
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to provide some additional functionality. Please note that the file formats
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created by ``gdbm`` and ``dbm`` are incompatible.
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The :mod:`gdbm` module provides an interface to the GNU DBM library. ``gdbm``
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objects behave like mappings (dictionaries), except that keys and values are
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always strings. Printing a ``gdbm`` object doesn't print the keys and values,
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and the :meth:`items` and :meth:`values` methods are not supported.
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The module defines the following constant and functions:
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.. exception:: error
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Raised on ``gdbm``\ -specific errors, such as I/O errors. :exc:`KeyError` is
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raised for general mapping errors like specifying an incorrect key.
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.. function:: open(filename, [flag, [mode]])
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Open a ``gdbm`` database and return a ``gdbm`` object. The *filename* argument
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is the name of the database file.
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The optional *flag* argument can be:
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+---------+-------------------------------------------+
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| Value | Meaning |
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+=========+===========================================+
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| ``'r'`` | Open existing database for reading only |
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| | (default) |
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+---------+-------------------------------------------+
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| ``'w'`` | Open existing database for reading and |
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| | writing |
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+---------+-------------------------------------------+
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| ``'c'`` | Open database for reading and writing, |
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| | creating it if it doesn't exist |
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+---------+-------------------------------------------+
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| ``'n'`` | Always create a new, empty database, open |
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| | for reading and writing |
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+---------+-------------------------------------------+
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The following additional characters may be appended to the flag to control
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how the database is opened:
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+---------+--------------------------------------------+
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| Value | Meaning |
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+=========+============================================+
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| ``'f'`` | Open the database in fast mode. Writes |
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| | to the database will not be synchronized. |
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+---------+--------------------------------------------+
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| ``'s'`` | Synchronized mode. This will cause changes |
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| | to the database to be immediately written |
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| | to the file. |
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+---------+--------------------------------------------+
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| ``'u'`` | Do not lock database. |
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+---------+--------------------------------------------+
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Not all flags are valid for all versions of ``gdbm``. The module constant
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:const:`open_flags` is a string of supported flag characters. The exception
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:exc:`error` is raised if an invalid flag is specified.
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The optional *mode* argument is the Unix mode of the file, used only when the
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database has to be created. It defaults to octal ``0666``.
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In addition to the dictionary-like methods, ``gdbm`` objects have the following
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methods:
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.. function:: firstkey()
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It's possible to loop over every key in the database using this method and the
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:meth:`nextkey` method. The traversal is ordered by ``gdbm``'s internal hash
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values, and won't be sorted by the key values. This method returns the starting
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key.
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.. function:: nextkey(key)
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Returns the key that follows *key* in the traversal. The following code prints
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every key in the database ``db``, without having to create a list in memory that
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contains them all::
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k = db.firstkey()
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while k != None:
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print(k)
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k = db.nextkey(k)
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.. function:: reorganize()
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If you have carried out a lot of deletions and would like to shrink the space
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used by the ``gdbm`` file, this routine will reorganize the database. ``gdbm``
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will not shorten the length of a database file except by using this
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reorganization; otherwise, deleted file space will be kept and reused as new
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(key, value) pairs are added.
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.. function:: sync()
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When the database has been opened in fast mode, this method forces any
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unwritten data to be written to the disk.
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.. seealso::
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Module :mod:`anydbm`
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Generic interface to ``dbm``\ -style databases.
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Module :mod:`whichdb`
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Utility module used to determine the type of an existing database.
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