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3.18copy -- Shallow and deep copy operations

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

Interface summary:

import copyx = copy.copy(y)        # make a shallow copy of yx = copy.deepcopy(y)    # make a deep copy of y

The difference between shallow and deep copying is only relevant forcompound objects (objects that contain other objects, like lists orclass instances):

• Ashallow copy constructs a new compound object and then (to theextent possible) insertsreferences into it to the objects foundin the original.

• Adeep copy constructs a new compound object and then,recursively, insertscopies into it of the objects found in theoriginal.

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

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

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

Thedeepcopy() function avoids these problems by:

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

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

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

Classes can use the same interfaces to control copying that they useto control pickling: they can define methods called__getinitargs__(),__getstate__() and__setstate__(). See the description of modulepickle for information on thesemethods. Thecopy module does not use thecopy_reg registration module. 

In order for a class to define its own copy implementation, it candefine special methods__copy__() and__deepcopy__(). The former is called to implement theshallow copy operation; no additional arguments are passed. Thelatter is called to implement the deep copy operation; it is passedone argument, the memo dictionary. If the__deepcopy__()implementation needs to make a deep copy of a component, it shouldcall thedeepcopy() function with the component as firstargument and the memo dictionary as second argument. 

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Python Library Reference

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Release 2.2.3, documentation updated on 30 May 2003.