
``shelve`` --- Python object persistence
****************************************

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
``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.

shelve.open(filename[, flag='c'[, protocol=None[, writeback=False]]])

   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 ``dbm.open()``.

   By default, version 3 pickles are used to serialize values.  The
   version of the pickle protocol can be specified with the *protocol*
   parameter.

   Because of Python semantics, a shelf cannot know when a mutable
   persistent-dictionary entry is modified.  By default modified
   objects are written *only* when assigned to the shelf (see
   *Example*).  If the optional *writeback* parameter is set to
   *True*, all entries accessed are also cached in memory, and written
   back on ``sync()`` and ``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).

   Note: Do not rely on the shelf being closed automatically; always call
     ``close()`` explicitly when you don't need it any more, or use a
     ``with`` statement with ``contextlib.closing()``.

Warning: Because the ``shelve`` module is backed by ``pickle``, it is
  insecure to load a shelf from an untrusted source.  Like with
  pickle, loading a shelf can execute arbitrary code.

Shelf objects support all methods supported by dictionaries.  This
eases the transition from dictionary based scripts to those requiring
persistent storage.

Two additional methods are supported:

Shelf.sync()

   Write back all entries in the cache if the shelf was opened with
   *writeback* set to ``True``.  Also empty the cache and synchronize
   the persistent dictionary on disk, if feasible.  This is called
   automatically when the shelf is closed with ``close()``.

Shelf.close()

   Synchronize and close the persistent *dict* object.  Operations on
   a closed shelf will fail with a ``ValueError``.

See also:

   Persistent dictionary recipe with widely supported storage formats
   and having the speed of native dictionaries.


Restrictions
============

* The choice of which database package will be used (such as
  ``dbm.ndbm`` or ``dbm.gnu``) depends on which interface is
  available.  Therefore it is not safe to open the database directly
  using ``dbm``.  The database is also (unfortunately) subject to the
  limitations of ``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 ``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.

class class shelve.Shelf(dict[, protocol=None[, writeback=False]])

   A subclass of ``collections.MutableMapping`` 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 ``pickle`` documentation for a discussion of the
   pickle protocols.

   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.

class class shelve.BsdDbShelf(dict[, protocol=None[, writeback=False]])

   A subclass of ``Shelf`` which exposes ``first()``, ``next()``,
   ``previous()``, ``last()`` and ``set_location()`` which are
   available in the third-party ``bsddb`` module from pybsddb 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 ``bsddb.hashopen()``,
   ``bsddb.btopen()`` or ``bsddb.rnopen()``.  The optional *protocol*
   and *writeback* parameters have the same interpretation as for the
   ``Shelf`` class.

class class shelve.DbfilenameShelf(filename[, flag='c'[, protocol=None[, writeback=False]]])

   A subclass of ``Shelf`` which accepts a *filename* instead of a
   dict-like object.  The underlying file will be opened using
   ``dbm.open()``.  By default, the file will be created and opened
   for both read and write.  The optional *flag* parameter has the
   same interpretation as for the ``open()`` function.  The optional
   *protocol* and *writeback* parameters have the same interpretation
   as for the ``Shelf`` class.


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 = key in d        # true if the key exists
   klist = list(d.keys()) # a list of all existing keys (slow!)

   # as d was opened WITHOUT writeback=True, beware:
   d['xx'] = [0, 1, 2]    # this works as expected, but...
   d['xx'].append(3)      # *this doesn't!* -- d['xx'] is STILL [0, 1, 2]!

   # 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

See also:

   Module ``dbm``
      Generic interface to ``dbm``-style databases.

   Module ``pickle``
      Object serialization used by ``shelve``.
