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214 lines
8.5 KiB
ReStructuredText
214 lines
8.5 KiB
ReStructuredText
:mod:`shelve` --- Python object persistence
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===========================================
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.. module:: shelve
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:synopsis: Python object persistence.
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**Source code:** :source:`Lib/shelve.py`
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.. index:: module: pickle
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--------------
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A "shelf" is a persistent, dictionary-like object. The difference with "dbm"
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databases is that the values (not the keys!) in a shelf can be essentially
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arbitrary Python objects --- anything that the :mod:`pickle` module can handle.
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This includes most class instances, recursive data types, and objects containing
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lots of shared sub-objects. The keys are ordinary strings.
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.. function:: open(filename, flag='c', protocol=None, writeback=False)
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Open a persistent dictionary. The filename specified is the base filename for
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the underlying database. As a side-effect, an extension may be added to the
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filename and more than one file may be created. By default, the underlying
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database file is opened for reading and writing. The optional *flag* parameter
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has the same interpretation as the *flag* parameter of :func:`dbm.open`.
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By default, pickles created with :data:`pickle.DEFAULT_PROTOCOL` are used
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to serialize values. The version of the pickle protocol can be specified
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with the *protocol* parameter.
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Because of Python semantics, a shelf cannot know when a mutable
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persistent-dictionary entry is modified. By default modified objects are
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written *only* when assigned to the shelf (see :ref:`shelve-example`). If the
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optional *writeback* parameter is set to ``True``, all entries accessed are also
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cached in memory, and written back on :meth:`~Shelf.sync` and
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:meth:`~Shelf.close`; this can make it handier to mutate mutable entries in
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the persistent dictionary, but, if many entries are accessed, it can consume
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vast amounts of memory for the cache, and it can make the close operation
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very slow since all accessed entries are written back (there is no way to
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determine which accessed entries are mutable, nor which ones were actually
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mutated).
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.. versionchanged:: 3.10
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:data:`pickle.DEFAULT_PROTOCOL` is now used as the default pickle
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protocol.
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.. note::
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Do not rely on the shelf being closed automatically; always call
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:meth:`~Shelf.close` explicitly when you don't need it any more, or
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use :func:`shelve.open` as a context manager::
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with shelve.open('spam') as db:
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db['eggs'] = 'eggs'
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.. warning::
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Because the :mod:`shelve` module is backed by :mod:`pickle`, it is insecure
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to load a shelf from an untrusted source. Like with pickle, loading a shelf
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can execute arbitrary code.
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Shelf objects support all methods supported by dictionaries. This eases the
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transition from dictionary based scripts to those requiring persistent storage.
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Two additional methods are supported:
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.. method:: Shelf.sync()
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Write back all entries in the cache if the shelf was opened with *writeback*
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set to :const:`True`. Also empty the cache and synchronize the persistent
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dictionary on disk, if feasible. This is called automatically when the shelf
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is closed with :meth:`close`.
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.. method:: Shelf.close()
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Synchronize and close the persistent *dict* object. Operations on a closed
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shelf will fail with a :exc:`ValueError`.
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.. seealso::
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`Persistent dictionary recipe <https://code.activestate.com/recipes/576642/>`_
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with widely supported storage formats and having the speed of native
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dictionaries.
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Restrictions
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------------
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.. index::
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module: dbm.ndbm
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module: dbm.gnu
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* The choice of which database package will be used (such as :mod:`dbm.ndbm` or
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:mod:`dbm.gnu`) depends on which interface is available. Therefore it is not
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safe to open the database directly using :mod:`dbm`. The database is also
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(unfortunately) subject to the limitations of :mod:`dbm`, if it is used ---
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this means that (the pickled representation of) the objects stored in the
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database should be fairly small, and in rare cases key collisions may cause
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the database to refuse updates.
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* The :mod:`shelve` module does not support *concurrent* read/write access to
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shelved objects. (Multiple simultaneous read accesses are safe.) When a
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program has a shelf open for writing, no other program should have it open for
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reading or writing. Unix file locking can be used to solve this, but this
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differs across Unix versions and requires knowledge about the database
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implementation used.
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.. class:: Shelf(dict, protocol=None, writeback=False, keyencoding='utf-8')
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A subclass of :class:`collections.abc.MutableMapping` which stores pickled
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values in the *dict* object.
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By default, pickles created with :data:`pickle.DEFAULT_PROTOCOL` are used
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to serialize values. The version of the pickle protocol can be specified
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with the *protocol* parameter. See the :mod:`pickle` documentation for a
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discussion of the pickle protocols.
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If the *writeback* parameter is ``True``, the object will hold a cache of all
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entries accessed and write them back to the *dict* at sync and close times.
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This allows natural operations on mutable entries, but can consume much more
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memory and make sync and close take a long time.
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The *keyencoding* parameter is the encoding used to encode keys before they
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are used with the underlying dict.
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A :class:`Shelf` object can also be used as a context manager, in which
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case it will be automatically closed when the :keyword:`with` block ends.
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.. versionchanged:: 3.2
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Added the *keyencoding* parameter; previously, keys were always encoded in
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UTF-8.
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.. versionchanged:: 3.4
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Added context manager support.
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.. versionchanged:: 3.10
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:data:`pickle.DEFAULT_PROTOCOL` is now used as the default pickle
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protocol.
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.. class:: BsdDbShelf(dict, protocol=None, writeback=False, keyencoding='utf-8')
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A subclass of :class:`Shelf` which exposes :meth:`first`, :meth:`!next`,
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:meth:`previous`, :meth:`last` and :meth:`set_location` which are available
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in the third-party :mod:`bsddb` module from `pybsddb
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<https://www.jcea.es/programacion/pybsddb.htm>`_ but not in other database
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modules. The *dict* object passed to the constructor must support those
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methods. This is generally accomplished by calling one of
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:func:`bsddb.hashopen`, :func:`bsddb.btopen` or :func:`bsddb.rnopen`. The
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optional *protocol*, *writeback*, and *keyencoding* parameters have the same
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interpretation as for the :class:`Shelf` class.
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.. class:: DbfilenameShelf(filename, flag='c', protocol=None, writeback=False)
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A subclass of :class:`Shelf` which accepts a *filename* instead of a dict-like
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object. The underlying file will be opened using :func:`dbm.open`. By
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default, the file will be created and opened for both read and write. The
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optional *flag* parameter has the same interpretation as for the :func:`.open`
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function. The optional *protocol* and *writeback* parameters have the same
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interpretation as for the :class:`Shelf` class.
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.. _shelve-example:
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Example
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-------
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To summarize the interface (``key`` is a string, ``data`` is an arbitrary
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object)::
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import shelve
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d = shelve.open(filename) # open -- file may get suffix added by low-level
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# library
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d[key] = data # store data at key (overwrites old data if
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# using an existing key)
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data = d[key] # retrieve a COPY of data at key (raise KeyError
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# if no such key)
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del d[key] # delete data stored at key (raises KeyError
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# if no such key)
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flag = key in d # true if the key exists
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klist = list(d.keys()) # a list of all existing keys (slow!)
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# as d was opened WITHOUT writeback=True, beware:
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d['xx'] = [0, 1, 2] # this works as expected, but...
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d['xx'].append(3) # *this doesn't!* -- d['xx'] is STILL [0, 1, 2]!
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# having opened d without writeback=True, you need to code carefully:
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temp = d['xx'] # extracts the copy
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temp.append(5) # mutates the copy
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d['xx'] = temp # stores the copy right back, to persist it
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# or, d=shelve.open(filename,writeback=True) would let you just code
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# d['xx'].append(5) and have it work as expected, BUT it would also
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# consume more memory and make the d.close() operation slower.
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d.close() # close it
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.. seealso::
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Module :mod:`dbm`
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Generic interface to ``dbm``-style databases.
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Module :mod:`pickle`
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Object serialization used by :mod:`shelve`.
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