copyreg — Register pickle support functions — Python 3.9.4 documentation

# `copyreg` — Register `pickle` support functions

Source code: Lib/copyreg.py

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.

`copyreg.` `constructor`(object)

  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=None)

  Declares that function should be used as a “reduction” function for objects of type type. function should return either a string or a tuple containing two or three elements.

  The optional constructor parameter, if provided, is a callable object which can be used to reconstruct the object when called with the tuple of arguments returned by function at pickling time. `TypeError` will be raised if object is a class or constructor is not callable.

  See the `pickle` module for more details on the interface expected of function and constructor. Note that the `dispatch_table` attribute of a pickler object or subclass of `pickle.Pickler` can also be used for declaring reduction functions.

## Example

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...