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5.17 copy -- Shallow and deep copy operations

5.17 copy -- Shallow and deep copy operations

This module provides generic (shallow and deep) copying operations.

Interface summary:

import copy

x = copy.copy(y)        # make a shallow copy of y
x = copy.deepcopy(y)    # make a deep copy of y
For module specific errors, copy.error is raised.

The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

Two problems often exist with deep copy operations that don't exist with shallow copy operations:

The deepcopy() function avoids these problems by:

This module does not copy types like module, method, stack trace, stack frame, file, socket, window, array, or any similar types. It does ``copy'' functions and classes (shallow and deeply), by returning the original object unchanged; this is compatible with the way these are treated by the pickle module. Changed in version 2.5: Added copying functions.

Classes can use the same interfaces to control copying that they use to control pickling. See the description of module pickle for information on these methods. The copy module does not use the copy_reg registration module.

In order for a class to define its own copy implementation, it can define special methods __copy__() and __deepcopy__(). The former is called to implement the shallow copy operation; no additional arguments are passed. The latter is called to implement the deep copy operation; it is passed one argument, the memo dictionary. If the __deepcopy__() implementation needs to make a deep copy of a component, it should call the deepcopy() function with the component as first argument and the memo dictionary as second argument.

See Also:

Module pickle:
Discussion of the special methods used to support object state retrieval and restoration.
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