2007-08-15 11:28:01 -03:00
|
|
|
:mod:`shelve` --- Python object persistence
|
|
|
|
===========================================
|
|
|
|
|
|
|
|
.. module:: shelve
|
|
|
|
:synopsis: Python object persistence.
|
|
|
|
|
|
|
|
|
|
|
|
.. index:: module: pickle
|
|
|
|
|
2011-08-18 21:14:03 -03:00
|
|
|
**Source code:** :source:`Lib/shelve.py`
|
|
|
|
|
|
|
|
--------------
|
|
|
|
|
2007-08-15 11:28:01 -03:00
|
|
|
A "shelf" is a persistent, dictionary-like object. The difference with "dbm"
|
|
|
|
databases is that the values (not the keys!) in a shelf can be essentially
|
|
|
|
arbitrary Python objects --- anything that the :mod:`pickle` module can handle.
|
|
|
|
This includes most class instances, recursive data types, and objects containing
|
|
|
|
lots of shared sub-objects. The keys are ordinary strings.
|
|
|
|
|
|
|
|
|
2012-05-22 05:27:40 -03:00
|
|
|
.. function:: open(filename, flag='c', protocol=None, writeback=False)
|
2007-08-15 11:28:01 -03:00
|
|
|
|
|
|
|
Open a persistent dictionary. The filename specified is the base filename for
|
|
|
|
the underlying database. As a side-effect, an extension may be added to the
|
|
|
|
filename and more than one file may be created. By default, the underlying
|
|
|
|
database file is opened for reading and writing. The optional *flag* parameter
|
|
|
|
has the same interpretation as the *flag* parameter of :func:`anydbm.open`.
|
|
|
|
|
|
|
|
By default, version 0 pickles are used to serialize values. The version of the
|
|
|
|
pickle protocol can be specified with the *protocol* parameter.
|
|
|
|
|
|
|
|
.. versionchanged:: 2.3
|
|
|
|
The *protocol* parameter was added.
|
|
|
|
|
2009-05-11 22:36:57 -03:00
|
|
|
Because of Python semantics, a shelf cannot know when a mutable
|
|
|
|
persistent-dictionary entry is modified. By default modified objects are
|
2010-02-10 21:38:42 -04:00
|
|
|
written *only* when assigned to the shelf (see :ref:`shelve-example`). If the
|
|
|
|
optional *writeback* parameter is set to *True*, all entries accessed are also
|
|
|
|
cached in memory, and written back on :meth:`~Shelf.sync` and
|
|
|
|
:meth:`~Shelf.close`; this can make it handier to mutate mutable entries in
|
|
|
|
the persistent dictionary, but, if many entries are accessed, it can consume
|
|
|
|
vast amounts of memory for the cache, and it can make the close operation
|
|
|
|
very slow since all accessed entries are written back (there is no way to
|
|
|
|
determine which accessed entries are mutable, nor which ones were actually
|
|
|
|
mutated).
|
2009-09-17 13:15:53 -03:00
|
|
|
|
2011-11-04 18:05:52 -03:00
|
|
|
Like file objects, shelve objects should be closed explicitly to ensure
|
|
|
|
that the persistent data is flushed to disk.
|
2009-09-17 13:15:53 -03:00
|
|
|
|
2013-09-22 06:43:10 -03:00
|
|
|
.. warning::
|
|
|
|
|
|
|
|
Because the :mod:`shelve` module is backed by :mod:`pickle`, it is insecure
|
|
|
|
to load a shelf from an untrusted source. Like with pickle, loading a shelf
|
|
|
|
can execute arbitrary code.
|
2007-08-15 11:28:01 -03:00
|
|
|
|
2016-09-12 00:38:27 -03:00
|
|
|
Shelf objects support most of the methods supported by dictionaries. This
|
|
|
|
eases the transition from dictionary based scripts to those requiring
|
|
|
|
persistent storage.
|
|
|
|
|
|
|
|
Note, the Python 3 transition methods (:meth:`~dict.viewkeys`,
|
|
|
|
:meth:`~dict.viewvalues`, and :meth:`~dict.viewitems`) are not supported.
|
2007-08-15 11:28:01 -03:00
|
|
|
|
2009-09-17 13:15:53 -03:00
|
|
|
Two additional methods are supported:
|
2007-08-15 11:28:01 -03:00
|
|
|
|
|
|
|
.. method:: Shelf.sync()
|
|
|
|
|
2009-09-17 13:15:53 -03:00
|
|
|
Write back all entries in the cache if the shelf was opened with *writeback*
|
|
|
|
set to :const:`True`. Also empty the cache and synchronize the persistent
|
|
|
|
dictionary on disk, if feasible. This is called automatically when the shelf
|
|
|
|
is closed with :meth:`close`.
|
|
|
|
|
|
|
|
.. method:: Shelf.close()
|
|
|
|
|
|
|
|
Synchronize and close the persistent *dict* object. Operations on a closed
|
|
|
|
shelf will fail with a :exc:`ValueError`.
|
|
|
|
|
2007-08-15 11:28:01 -03:00
|
|
|
|
2009-04-04 02:37:47 -03:00
|
|
|
.. seealso::
|
|
|
|
|
2016-02-26 14:37:12 -04:00
|
|
|
`Persistent dictionary recipe <https://code.activestate.com/recipes/576642/>`_
|
2009-04-04 02:37:47 -03:00
|
|
|
with widely supported storage formats and having the speed of native
|
|
|
|
dictionaries.
|
|
|
|
|
2007-08-15 11:28:01 -03:00
|
|
|
|
|
|
|
Restrictions
|
|
|
|
------------
|
|
|
|
|
|
|
|
.. index::
|
|
|
|
module: dbm
|
|
|
|
module: gdbm
|
|
|
|
module: bsddb
|
|
|
|
|
|
|
|
* The choice of which database package will be used (such as :mod:`dbm`,
|
|
|
|
:mod:`gdbm` or :mod:`bsddb`) depends on which interface is available. Therefore
|
|
|
|
it is not safe to open the database directly using :mod:`dbm`. The database is
|
|
|
|
also (unfortunately) subject to the limitations of :mod:`dbm`, if it is used ---
|
|
|
|
this means that (the pickled representation of) the objects stored in the
|
|
|
|
database should be fairly small, and in rare cases key collisions may cause the
|
|
|
|
database to refuse updates.
|
|
|
|
|
|
|
|
* The :mod:`shelve` module does not support *concurrent* read/write access to
|
|
|
|
shelved objects. (Multiple simultaneous read accesses are safe.) When a
|
|
|
|
program has a shelf open for writing, no other program should have it open for
|
|
|
|
reading or writing. Unix file locking can be used to solve this, but this
|
|
|
|
differs across Unix versions and requires knowledge about the database
|
|
|
|
implementation used.
|
|
|
|
|
|
|
|
|
2012-05-22 05:27:40 -03:00
|
|
|
.. class:: Shelf(dict, protocol=None, writeback=False)
|
2007-08-15 11:28:01 -03:00
|
|
|
|
|
|
|
A subclass of :class:`UserDict.DictMixin` which stores pickled values in the
|
|
|
|
*dict* object.
|
|
|
|
|
|
|
|
By default, version 0 pickles are used to serialize values. The version of the
|
|
|
|
pickle protocol can be specified with the *protocol* parameter. See the
|
|
|
|
:mod:`pickle` documentation for a discussion of the pickle protocols.
|
|
|
|
|
|
|
|
.. versionchanged:: 2.3
|
|
|
|
The *protocol* parameter was added.
|
|
|
|
|
|
|
|
If the *writeback* parameter is ``True``, the object will hold a cache of all
|
|
|
|
entries accessed and write them back to the *dict* at sync and close times.
|
|
|
|
This allows natural operations on mutable entries, but can consume much more
|
|
|
|
memory and make sync and close take a long time.
|
|
|
|
|
|
|
|
|
2012-05-22 05:27:40 -03:00
|
|
|
.. class:: BsdDbShelf(dict, protocol=None, writeback=False)
|
2007-08-15 11:28:01 -03:00
|
|
|
|
2009-07-26 11:19:57 -03:00
|
|
|
A subclass of :class:`Shelf` which exposes :meth:`first`, :meth:`!next`,
|
2007-08-15 11:28:01 -03:00
|
|
|
:meth:`previous`, :meth:`last` and :meth:`set_location` which are available in
|
|
|
|
the :mod:`bsddb` module but not in other database modules. The *dict* object
|
|
|
|
passed to the constructor must support those methods. This is generally
|
|
|
|
accomplished by calling one of :func:`bsddb.hashopen`, :func:`bsddb.btopen` or
|
|
|
|
:func:`bsddb.rnopen`. The optional *protocol* and *writeback* parameters have
|
|
|
|
the same interpretation as for the :class:`Shelf` class.
|
|
|
|
|
|
|
|
|
2012-05-22 05:27:40 -03:00
|
|
|
.. class:: DbfilenameShelf(filename, flag='c', protocol=None, writeback=False)
|
2007-08-15 11:28:01 -03:00
|
|
|
|
|
|
|
A subclass of :class:`Shelf` which accepts a *filename* instead of a dict-like
|
|
|
|
object. The underlying file will be opened using :func:`anydbm.open`. By
|
|
|
|
default, the file will be created and opened for both read and write. The
|
2009-07-26 11:19:57 -03:00
|
|
|
optional *flag* parameter has the same interpretation as for the :func:`.open`
|
2007-08-15 11:28:01 -03:00
|
|
|
function. The optional *protocol* and *writeback* parameters have the same
|
|
|
|
interpretation as for the :class:`Shelf` class.
|
|
|
|
|
|
|
|
|
2009-05-11 22:36:57 -03:00
|
|
|
.. _shelve-example:
|
|
|
|
|
2007-08-15 11:28:01 -03:00
|
|
|
Example
|
|
|
|
-------
|
|
|
|
|
|
|
|
To summarize the interface (``key`` is a string, ``data`` is an arbitrary
|
|
|
|
object)::
|
|
|
|
|
|
|
|
import shelve
|
|
|
|
|
|
|
|
d = shelve.open(filename) # open -- file may get suffix added by low-level
|
|
|
|
# library
|
|
|
|
|
|
|
|
d[key] = data # store data at key (overwrites old data if
|
|
|
|
# using an existing key)
|
|
|
|
data = d[key] # retrieve a COPY of data at key (raise KeyError if no
|
|
|
|
# such key)
|
|
|
|
del d[key] # delete data stored at key (raises KeyError
|
|
|
|
# if no such key)
|
|
|
|
flag = d.has_key(key) # true if the key exists
|
|
|
|
klist = d.keys() # a list of all existing keys (slow!)
|
|
|
|
|
|
|
|
# as d was opened WITHOUT writeback=True, beware:
|
|
|
|
d['xx'] = range(4) # this works as expected, but...
|
2009-04-05 07:32:26 -03:00
|
|
|
d['xx'].append(5) # *this doesn't!* -- d['xx'] is STILL range(4)!
|
2007-08-15 11:28:01 -03:00
|
|
|
|
|
|
|
# having opened d without writeback=True, you need to code carefully:
|
|
|
|
temp = d['xx'] # extracts the copy
|
|
|
|
temp.append(5) # mutates the copy
|
|
|
|
d['xx'] = temp # stores the copy right back, to persist it
|
|
|
|
|
|
|
|
# or, d=shelve.open(filename,writeback=True) would let you just code
|
|
|
|
# d['xx'].append(5) and have it work as expected, BUT it would also
|
|
|
|
# consume more memory and make the d.close() operation slower.
|
|
|
|
|
|
|
|
d.close() # close it
|
|
|
|
|
|
|
|
|
|
|
|
.. seealso::
|
|
|
|
|
|
|
|
Module :mod:`anydbm`
|
|
|
|
Generic interface to ``dbm``\ -style databases.
|
|
|
|
|
|
|
|
Module :mod:`bsddb`
|
|
|
|
BSD ``db`` database interface.
|
|
|
|
|
|
|
|
Module :mod:`dbhash`
|
2009-07-26 11:19:57 -03:00
|
|
|
Thin layer around the :mod:`bsddb` which provides an :func:`~dbhash.open`
|
|
|
|
function like the other database modules.
|
2007-08-15 11:28:01 -03:00
|
|
|
|
|
|
|
Module :mod:`dbm`
|
|
|
|
Standard Unix database interface.
|
|
|
|
|
|
|
|
Module :mod:`dumbdbm`
|
|
|
|
Portable implementation of the ``dbm`` interface.
|
|
|
|
|
|
|
|
Module :mod:`gdbm`
|
|
|
|
GNU database interface, based on the ``dbm`` interface.
|
|
|
|
|
|
|
|
Module :mod:`pickle`
|
|
|
|
Object serialization used by :mod:`shelve`.
|
|
|
|
|
|
|
|
Module :mod:`cPickle`
|
|
|
|
High-performance version of :mod:`pickle`.
|
|
|
|
|