12.3. "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 "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
     "shelve.open()" as a context manager:

        with shelve.open('spam') as db:
            db['eggs'] = 'eggs'

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.


12.3.1. 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 shelve.Shelf(dict, protocol=None, writeback=False, keyencoding='utf-8')

   A subclass of "collections.abc.MutableMapping" which stores pickled
   values in the *dict* object.

   By default, version 3 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.

   The *keyencoding* parameter is the encoding used to encode keys
   before they are used with the underlying dict.

   A "Shelf" object can also be used as a context manager, in which
   case it will be automatically closed when the "with" block ends.

   Changed in version 3.2: Added the *keyencoding* parameter;
   previously, keys were always encoded in UTF-8.

   Changed in version 3.4: Added context manager support.

class shelve.BsdDbShelf(dict, protocol=None, writeback=False, keyencoding='utf-8')

   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*, *writeback*, and *keyencoding* 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
   "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.


12.3.2. 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 = 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".
