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