= Python Copy =

'''`copy`''' is a module for deep- and shallow-copying operations. This is useful for any objects that are passed by reference, especially lists.

<<TableOfContents>>

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== Concept ==

A ''deep copy'' differs from a ''shallow copy'' only for containers. A shallow copy contains references to the same objects that the original contains. A deep copy contains copies of the original's contents.

This poses two challenges:

 1. Recursive objects may contain a reference to themselves, and causing a recursion loop
 2. Some data should be copied once and shared among the contents

Creating a deep copy then involves:

 1. Allowing objects to override the operation to avoid loops; see [[Python/DunderMethod#__DeepCopy__|__deepcopy__]] for details
 2. Keeping a `memo` dictionary for data already copied, so that it can be shared among the contents

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== Usage ==

Two functions are exposed: '''`copy()`''' and '''`deepcopy()`'''.

To create a deep copy, try:

{{{
import copy

class foo(object):
    def __init__(self, name):
        self.name = name

    def __eq__(self, other):
        return self.name == other.name

a = foo("foo")
lista = [a]
listb = lista

lista[0] == listb[0]  # True
lista[0] is listb[0]  # True

listc = copy.deepcopy(lista)

lista[0] == listc[0]  # True
lista[0] is listc[0]  # False
}}}

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== See also ==

[[https://docs.python.org/3/library/copy.html|Python copy module documentation]]

[[https://pymotw.com/3/copy/|Python Module of the Week article for copy]]



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