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Diffstat (limited to 'Doc/library/pickle.rst')
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diff --git a/Doc/library/pickle.rst b/Doc/library/pickle.rst index bf0a72e876..1b85bfadda 100644 --- a/Doc/library/pickle.rst +++ b/Doc/library/pickle.rst @@ -285,6 +285,29 @@ The :mod:`pickle` module exports two classes, :class:`Pickler` and See :ref:`pickle-persistent` for details and examples of uses. + .. attribute:: dispatch_table + + A pickler object's dispatch table is a registry of *reduction + functions* of the kind which can be declared using + :func:`copyreg.pickle`. It is a mapping whose keys are classes + and whose values are reduction functions. A reduction function + takes a single argument of the associated class and should + conform to the same interface as a :meth:`~object.__reduce__` + method. + + By default, a pickler object will not have a + :attr:`dispatch_table` attribute, and it will instead use the + global dispatch table managed by the :mod:`copyreg` module. + However, to customize the pickling for a specific pickler object + one can set the :attr:`dispatch_table` attribute to a dict-like + object. Alternatively, if a subclass of :class:`Pickler` has a + :attr:`dispatch_table` attribute then this will be used as the + default dispatch table for instances of that class. + + See :ref:`pickle-dispatch` for usage examples. + + .. versionadded:: 3.3 + .. attribute:: fast Deprecated. Enable fast mode if set to a true value. The fast mode @@ -575,6 +598,44 @@ pickle external objects by reference. .. literalinclude:: ../includes/dbpickle.py +.. _pickle-dispatch: + +Dispatch Tables +^^^^^^^^^^^^^^^ + +If one wants to customize pickling of some classes without disturbing +any other code which depends on pickling, then one can create a +pickler with a private dispatch table. + +The global dispatch table managed by the :mod:`copyreg` module is +available as :data:`copyreg.dispatch_table`. Therefore, one may +choose to use a modified copy of :data:`copyreg.dispatch_table` as a +private dispatch table. + +For example :: + + f = io.BytesIO() + p = pickle.Pickler(f) + p.dispatch_table = copyreg.dispatch_table.copy() + p.dispatch_table[SomeClass] = reduce_SomeClass + +creates an instance of :class:`pickle.Pickler` with a private dispatch +table which handles the ``SomeClass`` class specially. Alternatively, +the code :: + + class MyPickler(pickle.Pickler): + dispatch_table = copyreg.dispatch_table.copy() + dispatch_table[SomeClass] = reduce_SomeClass + f = io.BytesIO() + p = MyPickler(f) + +does the same, but all instances of ``MyPickler`` will by default +share the same dispatch table. The equivalent code using the +:mod:`copyreg` module is :: + + copyreg.pickle(SomeClass, reduce_SomeClass) + f = io.BytesIO() + p = pickle.Pickler(f) .. _pickle-state: |