python-clinic / Doc / library / shelve.rst

:mod:`shelve` --- Python object persistence

Source code: :source:`Lib/`

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 :mod:`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.


Because the :mod:`shelve` module is backed by :mod:`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:


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

A subclass of :class:`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 :mod:`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.

:class:`Shelf` objects can also be used as context managers.

A subclass of :class:`Shelf` which exposes :meth:`first`, :meth:`!next`, :meth:`previous`, :meth:`last` and :meth:`set_location` which are available in the third-party :mod:`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 :func:`bsddb.hashopen`, :func:`bsddb.btopen` or :func:`bsddb.rnopen`. The optional protocol, writeback, and keyencoding parameters have the same interpretation as for the :class:`Shelf` class.

A subclass of :class:`Shelf` which accepts a filename instead of a dict-like object. The underlying file will be opened using :func:``. 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 :func:`.open` function. The optional protocol and writeback parameters have the same interpretation as for the :class:`Shelf` class.


To summarize the interface (key is a string, data is an arbitrary object):

import shelve

d = # 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,,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