11.4. "shelve" — Python object persistence
******************************************

**Source code:** Lib/shelve.py

======================================================================

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 "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.

   Changed in version 2.3: The *protocol* parameter was added.

   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).

   Like file objects, shelve objects should be closed explicitly to
   ensure that the persistent data is flushed to disk.

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 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 ("viewkeys()", "viewvalues()",
and "viewitems()") are not supported.

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.


11.4.1. Restrictions
====================

* The choice of which database package will be used (such as "dbm",
  "gdbm" or "bsddb") 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 shelve.Shelf(dict, protocol=None, writeback=False)

   A subclass of "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 "pickle" documentation for a discussion of the
   pickle protocols.

   Changed in version 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.

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 "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 "bsddb.hashopen()",
   "bsddb.btopen()" or "bsddb.rnopen()".  The optional *protocol* and
   *writeback* parameters have the same interpretation as for the
   "Shelf" 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
   "anydbm.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.


11.4.2. 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...
   d['xx'].append(5)   # *this doesn't!* -- d['xx'] is STILL range(4)!

   # 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 "anydbm"
     Generic interface to "dbm"-style databases.

  Module "bsddb"
     BSD "db" database interface.

  Module "dbhash"
     Thin layer around the "bsddb" which provides an "open()" function
     like the other database modules.

  Module "dbm"
     Standard Unix database interface.

  Module "dumbdbm"
     Portable implementation of the "dbm" interface.

  Module "gdbm"
     GNU database interface, based on the "dbm" interface.

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

  Module "cPickle"
     High-performance version of "pickle".
