mirror of
https://github.com/python/cpython.git
synced 2025-11-04 03:44:55 +00:00
fa089b9b0b
Reformat header above separator line (added if missing) to a common format. Patch by Yoni Lavi.
356 lines
14 KiB
ReStructuredText
356 lines
14 KiB
ReStructuredText
: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 ---
|
|
:mod:`dbm.gnu` or :mod:`dbm.ndbm`. If none of these modules is 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.
|
|
|
|
|
|
.. 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.gnu`, :mod:`dbm.ndbm` or :mod:`dbm.dumb` --- should
|
|
be used to open a given file.
|
|
|
|
Returns 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; or a string containing the required module
|
|
name, such as ``'dbm.ndbm'`` or ``'dbm.gnu'``.
|
|
|
|
|
|
.. function:: open(file, flag='r', mode=0o666)
|
|
|
|
Open the database file *file* and return a corresponding object.
|
|
|
|
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 module listed above that can be imported is used.
|
|
|
|
The optional *flag* argument can be:
|
|
|
|
+---------+-------------------------------------------+
|
|
| Value | Meaning |
|
|
+=========+===========================================+
|
|
| ``'r'`` | Open existing database for reading only |
|
|
| | (default) |
|
|
+---------+-------------------------------------------+
|
|
| ``'w'`` | Open existing database for reading and |
|
|
| | writing |
|
|
+---------+-------------------------------------------+
|
|
| ``'c'`` | Open database for reading and writing, |
|
|
| | creating it if it doesn't exist |
|
|
+---------+-------------------------------------------+
|
|
| ``'n'`` | Always create a new, empty database, open |
|
|
| | for reading and writing |
|
|
+---------+-------------------------------------------+
|
|
|
|
The optional *mode* argument is the Unix mode of the file, used only when the
|
|
database has to be created. It defaults to octal ``0o666`` (and will be
|
|
modified by the prevailing umask).
|
|
|
|
|
|
The object returned by :func:`.open` supports the same basic functionality as
|
|
dictionaries; 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`.
|
|
|
|
.. versionchanged:: 3.2
|
|
:meth:`get` and :meth:`setdefault` are now available in all database modules.
|
|
|
|
Key and values are always stored as 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:: 3.4
|
|
Added native support for the context management protocol to the objects
|
|
returned by :func:`.open`.
|
|
|
|
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.gnu` --- GNU's reinterpretation of dbm
|
|
------------------------------------------------
|
|
|
|
.. module:: dbm.gnu
|
|
:platform: Unix
|
|
:synopsis: GNU's reinterpretation of dbm.
|
|
|
|
|
|
This module is quite similar to the :mod:`dbm` module, but uses the GNU library
|
|
``gdbm`` instead to provide some additional functionality. Please note that the
|
|
file formats created by :mod:`dbm.gnu` and :mod:`dbm.ndbm` are incompatible.
|
|
|
|
The :mod:`dbm.gnu` module provides an interface to the GNU DBM library.
|
|
``dbm.gnu.gdbm`` objects behave like mappings (dictionaries), except that keys and
|
|
values are always converted to bytes before storing. Printing a ``gdbm``
|
|
object doesn't print the
|
|
keys and values, and the :meth:`items` and :meth:`values` methods are not
|
|
supported.
|
|
|
|
.. 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[, mode]])
|
|
|
|
Open a ``gdbm`` database and return a :class:`gdbm` object. The *filename*
|
|
argument is the name of the database file.
|
|
|
|
The optional *flag* argument can be:
|
|
|
|
+---------+-------------------------------------------+
|
|
| Value | Meaning |
|
|
+=========+===========================================+
|
|
| ``'r'`` | Open existing database for reading only |
|
|
| | (default) |
|
|
+---------+-------------------------------------------+
|
|
| ``'w'`` | Open existing database for reading and |
|
|
| | writing |
|
|
+---------+-------------------------------------------+
|
|
| ``'c'`` | Open database for reading and writing, |
|
|
| | creating it if it doesn't exist |
|
|
+---------+-------------------------------------------+
|
|
| ``'n'`` | Always create a new, empty database, open |
|
|
| | for reading and writing |
|
|
+---------+-------------------------------------------+
|
|
|
|
The following additional characters may be appended to the flag to control
|
|
how the database is opened:
|
|
|
|
+---------+--------------------------------------------+
|
|
| Value | Meaning |
|
|
+=========+============================================+
|
|
| ``'f'`` | Open the database in fast mode. Writes |
|
|
| | to the database will not be synchronized. |
|
|
+---------+--------------------------------------------+
|
|
| ``'s'`` | Synchronized mode. This will cause changes |
|
|
| | to the database to be immediately written |
|
|
| | to the file. |
|
|
+---------+--------------------------------------------+
|
|
| ``'u'`` | Do not lock database. |
|
|
+---------+--------------------------------------------+
|
|
|
|
Not all flags are valid for all versions of ``gdbm``. The module constant
|
|
:const:`open_flags` is a string of supported flag characters. The exception
|
|
:exc:`error` is raised if an invalid flag is specified.
|
|
|
|
The optional *mode* argument is the Unix mode of the file, used only when the
|
|
database has to be created. It defaults to octal ``0o666``.
|
|
|
|
In addition to the dictionary-like methods, ``gdbm`` objects have the
|
|
following methods:
|
|
|
|
.. 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 != 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. ``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.
|
|
|
|
:mod:`dbm.ndbm` --- Interface based on ndbm
|
|
-------------------------------------------
|
|
|
|
.. module:: dbm.ndbm
|
|
:platform: Unix
|
|
:synopsis: The standard "database" interface, based on ndbm.
|
|
|
|
|
|
The :mod:`dbm.ndbm` module provides an interface to the Unix "(n)dbm" library.
|
|
Dbm objects behave like mappings (dictionaries), except that keys and values are
|
|
always stored as bytes. Printing a ``dbm`` object doesn't print the keys and
|
|
values, and the :meth:`items` and :meth:`values` methods are not supported.
|
|
|
|
This module can be used with the "classic" ndbm interface or the GNU GDBM
|
|
compatibility interface. On Unix, the :program:`configure` script will attempt
|
|
to locate the appropriate header file to simplify building this module.
|
|
|
|
.. 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[, mode]])
|
|
|
|
Open a dbm database and return a ``ndbm`` object. The *filename* argument is the
|
|
name of the database file (without the :file:`.dir` or :file:`.pag` extensions).
|
|
|
|
The optional *flag* argument must be one of these values:
|
|
|
|
+---------+-------------------------------------------+
|
|
| Value | Meaning |
|
|
+=========+===========================================+
|
|
| ``'r'`` | Open existing database for reading only |
|
|
| | (default) |
|
|
+---------+-------------------------------------------+
|
|
| ``'w'`` | Open existing database for reading and |
|
|
| | writing |
|
|
+---------+-------------------------------------------+
|
|
| ``'c'`` | Open database for reading and writing, |
|
|
| | creating it if it doesn't exist |
|
|
+---------+-------------------------------------------+
|
|
| ``'n'`` | Always create a new, empty database, open |
|
|
| | for reading and writing |
|
|
+---------+-------------------------------------------+
|
|
|
|
The optional *mode* argument is the Unix mode of the file, used only when the
|
|
database has to be created. It defaults to octal ``0o666`` (and will be
|
|
modified by the prevailing umask).
|
|
|
|
In addition to the dictionary-like methods, ``ndbm`` objects
|
|
provide the following method:
|
|
|
|
.. method:: ndbm.close()
|
|
|
|
Close the ``ndbm`` database.
|
|
|
|
|
|
:mod:`dbm.dumb` --- Portable DBM implementation
|
|
-----------------------------------------------
|
|
|
|
.. module:: dbm.dumb
|
|
:synopsis: Portable implementation of the simple DBM interface.
|
|
|
|
.. 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 dictionary-like interface which
|
|
is written entirely in Python. Unlike other modules such as :mod:`dbm.gnu` no
|
|
external library is required. As with other persistent mappings, the keys and
|
|
values are always stored as bytes.
|
|
|
|
The 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[, mode]])
|
|
|
|
Open a ``dumbdbm`` database and return a dumbdbm object. The *filename* argument is
|
|
the basename of the database file (without any specific extensions). When a
|
|
dumbdbm database is created, files with :file:`.dat` and :file:`.dir` extensions
|
|
are created.
|
|
|
|
The optional *flag* argument supports only the semantics of ``'c'``
|
|
and ``'n'`` values. Other values will default to database being always
|
|
opened for update, and will be created if it does not exist.
|
|
|
|
The optional *mode* argument is the Unix mode of the file, used only when the
|
|
database has to be created. It defaults to octal ``0o666`` (and will be modified
|
|
by the prevailing umask).
|
|
|
|
.. versionchanged:: 3.5
|
|
:func:`.open` always creates a new database when the flag has the value
|
|
``'n'``.
|
|
|
|
In addition to the methods provided by the
|
|
:class:`collections.abc.MutableMapping` class, :class:`dumbdbm` objects
|
|
provide the following methods:
|
|
|
|
.. 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 ``dumbdbm`` database.
|
|
|