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462 lines
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ReStructuredText
462 lines
14 KiB
ReStructuredText
:mod:`!dbm` --- Interfaces to Unix "databases"
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==============================================
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.. module:: dbm
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:synopsis: Interfaces to various Unix "database" formats.
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**Source code:** :source:`Lib/dbm/__init__.py`
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--------------
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:mod:`dbm` is a generic interface to variants of the DBM database:
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* :mod:`dbm.sqlite3`
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* :mod:`dbm.gnu`
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* :mod:`dbm.ndbm`
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If none of these modules are installed, the
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slow-but-simple implementation in module :mod:`dbm.dumb` will be used. There
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is a `third party interface <https://www.jcea.es/programacion/pybsddb.htm>`_ to
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the Oracle Berkeley DB.
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.. exception:: error
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A tuple containing the exceptions that can be raised by each of the supported
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modules, with a unique exception also named :exc:`dbm.error` as the first
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item --- the latter is used when :exc:`dbm.error` is raised.
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.. function:: whichdb(filename)
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This function attempts to guess which of the several simple database modules
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available --- :mod:`dbm.sqlite3`, :mod:`dbm.gnu`, :mod:`dbm.ndbm`,
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or :mod:`dbm.dumb` --- should be used to open a given file.
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Return one of the following values:
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* ``None`` if the file can't be opened because it's unreadable or doesn't exist
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* the empty string (``''``) if the file's format can't be guessed
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* a string containing the required module name, such as ``'dbm.ndbm'`` or ``'dbm.gnu'``
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.. versionchanged:: 3.11
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*filename* accepts a :term:`path-like object`.
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.. Substitutions for the open() flag param docs;
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all submodules use the same text.
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.. |flag_r| replace::
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Open existing database for reading only.
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.. |flag_w| replace::
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Open existing database for reading and writing.
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.. |flag_c| replace::
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Open database for reading and writing, creating it if it doesn't exist.
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.. |flag_n| replace::
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Always create a new, empty database, open for reading and writing.
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.. |mode_param_doc| replace::
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The Unix file access mode of the file (default: octal ``0o666``),
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used only when the database has to be created.
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.. function:: open(file, flag='r', mode=0o666)
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Open a database and return the corresponding database object.
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:param file:
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The database file to open.
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If the database file already exists, the :func:`whichdb` function is used to
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determine its type and the appropriate module is used; if it does not exist,
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the first submodule listed above that can be imported is used.
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:type file: :term:`path-like object`
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:param str flag:
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* ``'r'`` (default): |flag_r|
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* ``'w'``: |flag_w|
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* ``'c'``: |flag_c|
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* ``'n'``: |flag_n|
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:param int mode:
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|mode_param_doc|
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.. versionchanged:: 3.11
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*file* accepts a :term:`path-like object`.
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The object returned by :func:`~dbm.open` supports the same basic functionality as a
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:class:`dict`; keys and their corresponding values can be stored, retrieved, and
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deleted, and the :keyword:`in` operator and the :meth:`!keys` method are
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available, as well as :meth:`!get` and :meth:`!setdefault` methods.
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Key and values are always stored as :class:`bytes`. This means that when
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strings are used they are implicitly converted to the default encoding before
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being stored.
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These objects also support being used in a :keyword:`with` statement, which
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will automatically close them when done.
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.. versionchanged:: 3.2
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:meth:`!get` and :meth:`!setdefault` methods are now available for all
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:mod:`dbm` backends.
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.. versionchanged:: 3.4
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Added native support for the context management protocol to the objects
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returned by :func:`~dbm.open`.
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.. versionchanged:: 3.8
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Deleting a key from a read-only database raises a database module specific exception
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instead of :exc:`KeyError`.
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The following example records some hostnames and a corresponding title, and
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then prints out the contents of the database::
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import dbm
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# Open database, creating it if necessary.
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with dbm.open('cache', 'c') as db:
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# Record some values
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db[b'hello'] = b'there'
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db['www.python.org'] = 'Python Website'
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db['www.cnn.com'] = 'Cable News Network'
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# Note that the keys are considered bytes now.
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assert db[b'www.python.org'] == b'Python Website'
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# Notice how the value is now in bytes.
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assert db['www.cnn.com'] == b'Cable News Network'
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# Often-used methods of the dict interface work too.
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print(db.get('python.org', b'not present'))
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# Storing a non-string key or value will raise an exception (most
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# likely a TypeError).
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db['www.yahoo.com'] = 4
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# db is automatically closed when leaving the with statement.
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.. seealso::
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Module :mod:`shelve`
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Persistence module which stores non-string data.
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The individual submodules are described in the following sections.
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:mod:`dbm.sqlite3` --- SQLite backend for dbm
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---------------------------------------------
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.. module:: dbm.sqlite3
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:platform: All
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:synopsis: SQLite backend for dbm
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.. versionadded:: 3.13
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**Source code:** :source:`Lib/dbm/sqlite3.py`
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--------------
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This module uses the standard library :mod:`sqlite3` module to provide an
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SQLite backend for the :mod:`dbm` module.
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The files created by :mod:`dbm.sqlite3` can thus be opened by :mod:`sqlite3`,
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or any other SQLite browser, including the SQLite CLI.
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.. include:: ../includes/wasm-notavail.rst
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.. function:: open(filename, /, flag="r", mode=0o666)
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Open an SQLite database.
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The returned object behaves like a :term:`mapping`,
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implements a :meth:`!close` method,
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and supports a "closing" context manager via the :keyword:`with` keyword.
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:param filename:
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The path to the database to be opened.
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:type filename: :term:`path-like object`
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:param str flag:
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* ``'r'`` (default): |flag_r|
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* ``'w'``: |flag_w|
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* ``'c'``: |flag_c|
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* ``'n'``: |flag_n|
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:param mode:
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The Unix file access mode of the file (default: octal ``0o666``),
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used only when the database has to be created.
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:mod:`dbm.gnu` --- GNU database manager
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---------------------------------------
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.. module:: dbm.gnu
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:platform: Unix
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:synopsis: GNU database manager
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**Source code:** :source:`Lib/dbm/gnu.py`
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--------------
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The :mod:`dbm.gnu` module provides an interface to the :abbr:`GDBM (GNU dbm)`
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library, similar to the :mod:`dbm.ndbm` module, but with additional
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functionality like crash tolerance.
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.. note::
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The file formats created by :mod:`dbm.gnu` and :mod:`dbm.ndbm` are incompatible
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and can not be used interchangeably.
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.. include:: ../includes/wasm-mobile-notavail.rst
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.. exception:: error
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Raised on :mod:`dbm.gnu`-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="r", mode=0o666, /)
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Open a GDBM database and return a :class:`!gdbm` object.
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:param filename:
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The database file to open.
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:type filename: :term:`path-like object`
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:param str flag:
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* ``'r'`` (default): |flag_r|
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* ``'w'``: |flag_w|
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* ``'c'``: |flag_c|
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* ``'n'``: |flag_n|
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The following additional characters may be appended
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to control how the database is opened:
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* ``'f'``: Open the database in fast mode.
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Writes to the database will not be synchronized.
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* ``'s'``: Synchronized mode.
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Changes to the database will be written immediately to the file.
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* ``'u'``: Do not lock database.
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Not all flags are valid for all versions of GDBM.
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See the :data:`open_flags` member for a list of supported flag characters.
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:param int mode:
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|mode_param_doc|
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:raises error:
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If an invalid *flag* argument is passed.
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.. versionchanged:: 3.11
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*filename* accepts a :term:`path-like object`.
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.. data:: open_flags
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A string of characters the *flag* parameter of :meth:`~dbm.gnu.open` supports.
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:class:`!gdbm` objects behave similar to :term:`mappings <mapping>`,
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but :meth:`!items` and :meth:`!values` methods are not supported.
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The following methods are also provided:
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.. method:: gdbm.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
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hash values, and won't be sorted by the key values. This method returns
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the starting key.
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.. method:: gdbm.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 is not None:
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print(k)
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k = db.nextkey(k)
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.. method:: gdbm.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. :class:`!gdbm`
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objects 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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.. method:: gdbm.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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.. method:: gdbm.close()
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Close the GDBM database.
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.. method:: gdbm.clear()
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Remove all items from the GDBM database.
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.. versionadded:: 3.13
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:mod:`dbm.ndbm` --- New Database Manager
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----------------------------------------
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.. module:: dbm.ndbm
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:platform: Unix
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:synopsis: The New Database Manager
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**Source code:** :source:`Lib/dbm/ndbm.py`
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--------------
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The :mod:`dbm.ndbm` module provides an interface to the
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:abbr:`NDBM (New Database Manager)` library.
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This module can be used with the "classic" NDBM interface or the
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:abbr:`GDBM (GNU dbm)` compatibility interface.
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.. note::
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The file formats created by :mod:`dbm.gnu` and :mod:`dbm.ndbm` are incompatible
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and can not be used interchangeably.
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.. warning::
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The NDBM library shipped as part of macOS has an undocumented limitation on the
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size of values, which can result in corrupted database files
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when storing values larger than this limit. Reading such corrupted files can
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result in a hard crash (segmentation fault).
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.. include:: ../includes/wasm-mobile-notavail.rst
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.. exception:: error
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Raised on :mod:`dbm.ndbm`-specific errors, such as I/O errors. :exc:`KeyError` is raised
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for general mapping errors like specifying an incorrect key.
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.. data:: library
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Name of the NDBM implementation library used.
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.. function:: open(filename, flag="r", mode=0o666, /)
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Open an NDBM database and return an :class:`!ndbm` object.
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:param filename:
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The basename of the database file
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(without the :file:`.dir` or :file:`.pag` extensions).
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:type filename: :term:`path-like object`
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:param str flag:
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* ``'r'`` (default): |flag_r|
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* ``'w'``: |flag_w|
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* ``'c'``: |flag_c|
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* ``'n'``: |flag_n|
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:param int mode:
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|mode_param_doc|
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:class:`!ndbm` objects behave similar to :term:`mappings <mapping>`,
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but :meth:`!items` and :meth:`!values` methods are not supported.
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The following methods are also provided:
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.. versionchanged:: 3.11
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Accepts :term:`path-like object` for filename.
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.. method:: ndbm.close()
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Close the NDBM database.
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.. method:: ndbm.clear()
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Remove all items from the NDBM database.
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.. versionadded:: 3.13
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:mod:`dbm.dumb` --- Portable DBM implementation
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-----------------------------------------------
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.. module:: dbm.dumb
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:synopsis: Portable implementation of the simple DBM interface.
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**Source code:** :source:`Lib/dbm/dumb.py`
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.. index:: single: databases
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.. note::
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The :mod:`dbm.dumb` module is intended as a last resort fallback for the
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:mod:`dbm` module when a more robust module is not available. The :mod:`dbm.dumb`
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module is not written for speed and is not nearly as heavily used as the other
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database modules.
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--------------
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The :mod:`dbm.dumb` module provides a persistent :class:`dict`-like
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interface which is written entirely in Python.
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Unlike other :mod:`dbm` backends, such as :mod:`dbm.gnu`, no
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external library is required.
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The :mod:`!dbm.dumb` module defines the following:
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.. exception:: error
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Raised on :mod:`dbm.dumb`-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="c", mode=0o666)
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Open a :mod:`!dbm.dumb` database.
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The returned database object behaves similar to a :term:`mapping`,
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in addition to providing :meth:`~dumbdbm.sync` and :meth:`~dumbdbm.close`
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methods.
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:param filename:
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The basename of the database file (without extensions).
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A new database creates the following files:
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- :file:`{filename}.dat`
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- :file:`{filename}.dir`
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:type database: :term:`path-like object`
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:param str flag:
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* ``'r'``: |flag_r|
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* ``'w'``: |flag_w|
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* ``'c'`` (default): |flag_c|
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* ``'n'``: |flag_n|
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:param int mode:
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|mode_param_doc|
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.. warning::
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It is possible to crash the Python interpreter when loading a database
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with a sufficiently large/complex entry due to stack depth limitations in
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Python's AST compiler.
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.. versionchanged:: 3.5
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:func:`~dbm.dumb.open` always creates a new database when *flag* is ``'n'``.
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.. versionchanged:: 3.8
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A database opened read-only if *flag* is ``'r'``.
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A database is not created if it does not exist if *flag* is ``'r'`` or ``'w'``.
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.. versionchanged:: 3.11
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*filename* accepts a :term:`path-like object`.
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In addition to the methods provided by the
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:class:`collections.abc.MutableMapping` class,
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the following methods are provided:
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.. method:: dumbdbm.sync()
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Synchronize the on-disk directory and data files. This method is called
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by the :meth:`shelve.Shelf.sync` method.
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.. method:: dumbdbm.close()
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Close the database.
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