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/Doc/library/shelve.rst

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  1:mod:`shelve` --- Python object persistence
  2===========================================
  3
  4.. module:: shelve
  5   :synopsis: Python object persistence.
  6
  7
  8.. index:: module: pickle
  9
 10A "shelf" is a persistent, dictionary-like object.  The difference with "dbm"
 11databases is that the values (not the keys!) in a shelf can be essentially
 12arbitrary Python objects --- anything that the :mod:`pickle` module can handle.
 13This includes most class instances, recursive data types, and objects containing
 14lots of shared  sub-objects.  The keys are ordinary strings.
 15
 16
 17.. function:: open(filename[, flag='c'[, protocol=None[, writeback=False]]])
 18
 19   Open a persistent dictionary.  The filename specified is the base filename for
 20   the underlying database.  As a side-effect, an extension may be added to the
 21   filename and more than one file may be created.  By default, the underlying
 22   database file is opened for reading and writing.  The optional *flag* parameter
 23   has the same interpretation as the *flag* parameter of :func:`anydbm.open`.
 24
 25   By default, version 0 pickles are used to serialize values.  The version of the
 26   pickle protocol can be specified with the *protocol* parameter.
 27
 28   .. versionchanged:: 2.3
 29      The *protocol* parameter was added.
 30
 31   Because of Python semantics, a shelf cannot know when a mutable
 32   persistent-dictionary entry is modified.  By default modified objects are
 33   written only when assigned to the shelf (see :ref:`shelve-example`).  If
 34   the optional *writeback* parameter is set to *True*, all entries accessed
 35   are cached in memory, and written back at close time; this can make it
 36   handier to mutate mutable entries in the persistent dictionary, but, if
 37   many entries are accessed, it can consume vast amounts of memory for the
 38   cache, and it can make the close operation very slow since all accessed
 39   entries are written back (there is no way to determine which accessed
 40   entries are mutable, nor which ones were actually mutated).
 41
 42Shelf objects support all methods supported by dictionaries.  This eases the
 43transition from dictionary based scripts to those requiring persistent storage.
 44
 45One additional method is supported:
 46
 47
 48.. method:: Shelf.sync()
 49
 50   Write back all entries in the cache if the shelf was opened with *writeback* set
 51   to *True*. Also empty the cache and synchronize the persistent dictionary on
 52   disk, if feasible.  This is called automatically when the shelf is closed with
 53   :meth:`close`.
 54
 55.. seealso::
 56
 57   `Persistent dictionary recipe <http://code.activestate.com/recipes/576642/>`_
 58   with widely supported storage formats and having the speed of native
 59   dictionaries.
 60
 61
 62Restrictions
 63------------
 64
 65  .. index::
 66     module: dbm
 67     module: gdbm
 68     module: bsddb
 69
 70* The choice of which database package will be used (such as :mod:`dbm`,
 71  :mod:`gdbm` or :mod:`bsddb`) depends on which interface is available.  Therefore
 72  it is not safe to open the database directly using :mod:`dbm`.  The database is
 73  also (unfortunately) subject to the limitations of :mod:`dbm`, if it is used ---
 74  this means that (the pickled representation of) the objects stored in the
 75  database should be fairly small, and in rare cases key collisions may cause the
 76  database to refuse updates.
 77
 78* Depending on the implementation, closing a persistent dictionary may or may
 79  not be necessary to flush changes to disk.  The :meth:`__del__` method of the
 80  :class:`Shelf` class calls the :meth:`close` method, so the programmer generally
 81  need not do this explicitly.
 82
 83* The :mod:`shelve` module does not support *concurrent* read/write access to
 84  shelved objects.  (Multiple simultaneous read accesses are safe.)  When a
 85  program has a shelf open for writing, no other program should have it open for
 86  reading or writing.  Unix file locking can be used to solve this, but this
 87  differs across Unix versions and requires knowledge about the database
 88  implementation used.
 89
 90
 91.. class:: Shelf(dict[, protocol=None[, writeback=False]])
 92
 93   A subclass of :class:`UserDict.DictMixin` which stores pickled values in the
 94   *dict* object.
 95
 96   By default, version 0 pickles are used to serialize values.  The version of the
 97   pickle protocol can be specified with the *protocol* parameter. See the
 98   :mod:`pickle` documentation for a discussion of the pickle protocols.
 99
100   .. versionchanged:: 2.3
101      The *protocol* parameter was added.
102
103   If the *writeback* parameter is ``True``, the object will hold a cache of all
104   entries accessed and write them back to the *dict* at sync and close times.
105   This allows natural operations on mutable entries, but can consume much more
106   memory and make sync and close take a long time.
107
108
109.. class:: BsdDbShelf(dict[, protocol=None[, writeback=False]])
110
111   A subclass of :class:`Shelf` which exposes :meth:`first`, :meth:`next`,
112   :meth:`previous`, :meth:`last` and :meth:`set_location` which are available in
113   the :mod:`bsddb` module but not in other database modules.  The *dict* object
114   passed to the constructor must support those methods.  This is generally
115   accomplished by calling one of :func:`bsddb.hashopen`, :func:`bsddb.btopen` or
116   :func:`bsddb.rnopen`.  The optional *protocol* and *writeback* parameters have
117   the same interpretation as for the :class:`Shelf` class.
118
119
120.. class:: DbfilenameShelf(filename[, flag='c'[, protocol=None[, writeback=False]]])
121
122   A subclass of :class:`Shelf` which accepts a *filename* instead of a dict-like
123   object.  The underlying file will be opened using :func:`anydbm.open`.  By
124   default, the file will be created and opened for both read and write.  The
125   optional *flag* parameter has the same interpretation as for the :func:`open`
126   function.  The optional *protocol* and *writeback* parameters have the same
127   interpretation as for the :class:`Shelf` class.
128
129
130.. _shelve-example:
131
132Example
133-------
134
135To summarize the interface (``key`` is a string, ``data`` is an arbitrary
136object)::
137
138   import shelve
139
140   d = shelve.open(filename) # open -- file may get suffix added by low-level
141                             # library
142
143   d[key] = data   # store data at key (overwrites old data if
144                   # using an existing key)
145   data = d[key]   # retrieve a COPY of data at key (raise KeyError if no
146                   # such key)
147   del d[key]      # delete data stored at key (raises KeyError
148                   # if no such key)
149   flag = d.has_key(key)   # true if the key exists
150   klist = d.keys() # a list of all existing keys (slow!)
151
152   # as d was opened WITHOUT writeback=True, beware:
153   d['xx'] = range(4)  # this works as expected, but...
154   d['xx'].append(5)   # *this doesn't!* -- d['xx'] is STILL range(4)!
155
156   # having opened d without writeback=True, you need to code carefully:
157   temp = d['xx']      # extracts the copy
158   temp.append(5)      # mutates the copy
159   d['xx'] = temp      # stores the copy right back, to persist it
160
161   # or, d=shelve.open(filename,writeback=True) would let you just code
162   # d['xx'].append(5) and have it work as expected, BUT it would also
163   # consume more memory and make the d.close() operation slower.
164
165   d.close()       # close it
166
167
168.. seealso::
169
170   Module :mod:`anydbm`
171      Generic interface to ``dbm``\ -style databases.
172
173   Module :mod:`bsddb`
174      BSD ``db`` database interface.
175
176   Module :mod:`dbhash`
177      Thin layer around the :mod:`bsddb` which provides an :func:`open` function like
178      the other database modules.
179
180   Module :mod:`dbm`
181      Standard Unix database interface.
182
183   Module :mod:`dumbdbm`
184      Portable implementation of the ``dbm`` interface.
185
186   Module :mod:`gdbm`
187      GNU database interface, based on the ``dbm`` interface.
188
189   Module :mod:`pickle`
190      Object serialization used by :mod:`shelve`.
191
192   Module :mod:`cPickle`
193      High-performance version of :mod:`pickle`.
194