Doc/library/dbm.rst
!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:
dbm.sqlite3dbm.gnudbm.ndbmIf 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 <https://www.jcea.es/programacion/pybsddb.htm>_ to
the Oracle Berkeley DB.
.. note::
None of the underlying modules will automatically shrink the disk space used by
the database file. However, :mod:dbm.sqlite3, :mod:dbm.gnu and :mod:dbm.dumb
provide a :meth:!reorganize method that can be used for this purpose.
.. 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'') if the file's format can't be guessed'dbm.ndbm' or 'dbm.gnu'.. versionchanged:: 3.11
filename accepts a :term:path-like object.
.. Substitutions for the open() flag param docs; all submodules use the same text.
.. |flag_r| replace:: Open existing database for reading only.
.. |flag_w| replace:: Open existing database for reading and writing.
.. |flag_c| replace:: Open database for reading and writing, creating it if it doesn't exist.
.. |flag_n| replace:: Always create a new, empty database, open for reading and writing.
.. |mode_param_doc| replace::
The Unix file access mode of the file (default: octal 0o666),
used only when the database has to be created.
.. 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 basic
functionality of mutable :term:mappings <mapping>;
keys and their corresponding values can be stored, retrieved, and
deleted, and iteration, the :keyword:in operator and methods :meth:!keys,
:meth:!get, :meth:!setdefault and :meth:!clear are available.
The :meth:!keys method returns a list instead of a view object.
The :meth:!setdefault method requires two arguments.
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.
.. versionchanged:: 3.13
:meth:!clear methods are now available for all :mod:!dbm backends.
The following example records some hostnames and a corresponding title, and then prints out the contents of the database::
import dbm
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
.. seealso::
Module :mod:shelve
Persistence module which stores non-string data.
The individual submodules are described in the following sections.
!dbm.sqlite3 --- SQLite backend for dbm.. module:: dbm.sqlite3 :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.
.. include:: ../includes/wasm-notavail.rst
.. function:: open(filename, /, flag="r", mode=0o666)
Open an SQLite database.
: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.
The returned database object behaves similar to a mutable :term:mapping,
but the :meth:!keys method returns a list, and
the :meth:!setdefault method requires two arguments.
It also supports a "closing" context manager via the :keyword:with keyword.
The following methods are also provided:
.. method:: sqlite3.close()
Close the SQLite database.
.. method:: sqlite3.reorganize()
If you have carried out a lot of deletions and would like to shrink the space
used on disk, this method will reorganize the database; otherwise, deleted file
space will be kept and reused as new (key, value) pairs are added.
.. note::
While reorganizing, as much as two times the size of the original database is required
in free disk space. However, be aware that this factor changes for each :mod:`!dbm` submodule.
.. versionadded:: 3.15
!dbm.gnu --- GNU database manager.. module:: dbm.gnu :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.
.. include:: ../includes/wasm-mobile-notavail.rst
.. availability:: Unix.
.. 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.
.. data:: open_flags
A string of characters the flag parameter of :meth:~dbm.gnu.open supports.
.. 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.
:class:!gdbm objects behave similar to mutable :term:mappings <mapping>,
but methods :meth:!items, :meth:!values, :meth:!pop, :meth:!popitem,
and :meth:!update are not supported,
the :meth:!keys method returns a list, and
the :meth:!setdefault method requires two arguments.
It also supports a "closing" context manager via the :keyword:with keyword.
.. versionchanged:: 3.2
Added the :meth:!get and :meth:!setdefault methods.
.. versionchanged:: 3.13
Added the :meth:!clear method.
The following methods are also provided:
.. method:: gdbm.close()
Close the GDBM database.
.. 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.
.. note::
While reorganizing, as much as one time the size of the original database is required
in free disk space. However, be aware that this factor changes for each :mod:`!dbm` submodule.
.. method:: gdbm.sync()
When the database has been opened in fast mode, this method forces any
unwritten data to be written to the disk.
!dbm.ndbm --- New Database Manager.. module:: dbm.ndbm :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).
.. include:: ../includes/wasm-mobile-notavail.rst
.. availability:: Unix.
.. 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|
.. versionchanged:: 3.11
Accepts :term:path-like object for filename.
:class:!ndbm objects behave similar to mutable :term:mappings <mapping>,
but methods :meth:!items, :meth:!values, :meth:!pop, :meth:!popitem,
and :meth:!update are not supported,
the :meth:!keys method returns a list, and
the :meth:!setdefault method requires two arguments.
It also supports a "closing" context manager via the :keyword:with keyword.
.. versionchanged:: 3.2
Added the :meth:!get and :meth:!setdefault methods.
.. versionchanged:: 3.13
Added the :meth:!clear method.
The following method is also provided:
.. method:: ndbm.close()
Close the NDBM database.
!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.
: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.
.. warning::
:mod:!dbm.dumb does not support concurrent read/write access. (Multiple
simultaneous read accesses are safe.) When a program has the database open
for writing, no other program should have it open for reading or writing.
.. 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.
The returned database object behaves similar to a mutable :term:mapping,
but the :meth:!keys and :meth:!items methods return lists, and
the :meth:!setdefault method requires two arguments.
It also supports a "closing" context manager via the :keyword:with keyword.
The following methods are also provided:
.. method:: dumbdbm.close()
Close the database.
.. method:: dumbdbm.reorganize()
If you have carried out a lot of deletions and would like to shrink the space
used on disk, this method will reorganize the database; otherwise, deleted file
space will not be reused.
.. note::
While reorganizing, no additional free disk space is required. However, be aware
that this factor changes for each :mod:`!dbm` submodule.
.. versionadded:: 3.15
.. method:: dumbdbm.sync()
Synchronize the on-disk directory and data files. This method is called
by the :meth:`shelve.Shelf.sync` method.