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Python Library Reference

Previous: 3.17.2 Example Up: 3. Python Runtime Services Next: 3.19 marshal

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3.18 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

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

• A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.

• A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

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

• Recursive objects (compound objects that, directly or indirectly, contain a reference to themselves) may cause a recursive loop.

• Because deep copy copies everything it may copy too much, e.g., administrative data structures that should be shared even between copies.

The deepcopy() function avoids these problems by:

• keeping a ``memo'' dictionary of objects already copied during the current copying pass; and

• letting user-defined classes override the copying operation or the set of components copied.

This version does not copy types like module, class, function, method, stack trace, stack frame, file, socket, window, array, or any similar types.

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.

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Python Library Reference

Previous: 3.17.2 Example Up: 3. Python Runtime Services Next: 3.19 marshal

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Release 2.3.4, documentation updated on May 20, 2004.