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.

Example

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

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 __deepcopy__ for details

2. Keeping a memo dictionary for data already copied, so that it can be shared among the contents

Usage

Copy

Make a shallow copy of an object.

DeepCopy

Make a deep copy of an object.

See also

Python copy module documentation

Python Module of the Week article for copy

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