copyreg — Register pickle support functions

Source code: Lib/

The copyreg module offers a way to define functions used while pickling specific objects. The pickle and copy modules use those functions when pickling/copying those objects. The module provides configuration information about object constructors which are not classes. Such constructors may be factory functions or class instances.


Declares object to be a valid constructor. If object is not callable (and hence not valid as a constructor), raises TypeError.

copyreg.pickle(type, function, constructor_ob=None)

Declares that function should be used as a “reduction” function for objects of type type. function must return either a string or a tuple containing between two and six elements. See the dispatch_table for more details on the interface of function.

The constructor_ob parameter is a legacy feature and is now ignored, but if passed it must be a callable.

Note that the dispatch_table attribute of a pickler object or subclass of pickle.Pickler can also be used for declaring reduction functions.


The example below would like to show how to register a pickle function and how it will be used:

>>> import copyreg, copy, pickle
>>> class C:
...     def __init__(self, a):
...         self.a = a
>>> def pickle_c(c):
...     print("pickling a C instance...")
...     return C, (c.a,)
>>> copyreg.pickle(C, pickle_c)
>>> c = C(1)
>>> d = copy.copy(c)  
pickling a C instance...
>>> p = pickle.dumps(c)  
pickling a C instance...