:mod:`!shelve` --- Python object persistence
.. module:: shelve :synopsis: Python object persistence.
Source code: :source:`Lib/shelve.py`
.. index:: pair: module; pickle
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.
.. function:: open(filename, flag='c', protocol=None, writeback=False, *, \
serializer=None, deserializer=None)
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 :func:`dbm.open`.
By default, pickles created with :const:`pickle.DEFAULT_PROTOCOL` 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 :ref:`shelve-example`). If the
optional *writeback* parameter is set to ``True``, all entries accessed are also
cached in memory, and written back on :meth:`~Shelf.sync` and
:meth:`~Shelf.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).
By default, :mod:`!shelve` uses :func:`pickle.dumps` and :func:`pickle.loads`
for serializing and deserializing. This can be changed by supplying
*serializer* and *deserializer*, respectively.
The *serializer* argument must be a callable which takes an object ``obj``
and the *protocol* as inputs and returns the representation ``obj`` as a
:term:`bytes-like object`; the *protocol* value may be ignored by the
serializer.
The *deserializer* argument must be a callable which takes a serialized object
given as a :class:`bytes` object and returns the corresponding object.
A :exc:`ShelveError` is raised if *serializer* is given but *deserializer*
is not, or vice-versa.
.. versionchanged:: 3.10
:const:`pickle.DEFAULT_PROTOCOL` is now used as the default pickle
protocol.
.. versionchanged:: 3.11
Accepts :term:`path-like object` for filename.
.. versionchanged:: 3.15
Accepts custom *serializer* and *deserializer* functions in place of
:func:`pickle.dumps` and :func:`pickle.loads`.
.. note::
Do not rely on the shelf being closed automatically; always call
:meth:`~Shelf.close` explicitly when you don't need it any more, or
use :func:`shelve.open` as a context manager::
with shelve.open('spam') as db:
db['eggs'] = 'eggs'
Warning
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 most of the methods and operations supported by dictionaries
(except copying, constructors and operators | and |=). This eases the
transition from dictionary based scripts to those requiring persistent storage.
Two additional methods are supported:
.. method:: Shelf.sync() Write back all entries in the cache if the shelf was opened with *writeback* set to :const:`True`. Also empty the cache and synchronize the persistent dictionary on disk, if feasible. This is called automatically when :meth:`reorganize` is called or the shelf is closed with :meth:`close`.
.. method:: Shelf.reorganize() Calls :meth:`sync` and attempts to shrink space used on disk by removing empty space resulting from deletions. .. versionadded:: 3.15
.. method:: Shelf.close() Synchronize and close the persistent *dict* object. Operations on a closed shelf will fail with a :exc:`ValueError`.
.. seealso:: `Persistent dictionary recipe <https://code.activestate.com/recipes/576642-persistent-dict-with-multiple-standard-file-format/>`_ with widely supported storage formats and having the speed of native dictionaries.
.. index:: pair: module; dbm.ndbm pair: module; dbm.gnu
- 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.
- On macOS :mod:`dbm.ndbm` can silently corrupt the database file on updates, which can cause hard crashes when trying to read from the database.
- :meth:`Shelf.reorganize` may not be available for all database packages and may temporarily increase resource usage (especially disk space) when called. Additionally, it will never run automatically and instead needs to be called explicitly.
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
.. exception:: ShelveError Exception raised when one of the arguments *deserializer* and *serializer* is missing in the :func:`~shelve.open`, :class:`Shelf`, :class:`BsdDbShelf` and :class:`DbfilenameShelf`. The *deserializer* and *serializer* arguments must be given together. .. versionadded:: 3.15
.. seealso::
Module :mod:`dbm`
Generic interface to ``dbm``-style databases.
Module :mod:`pickle`
Object serialization used by :mod:`!shelve`.
