Python Standard Library

The copy Module

The copy module contains two functions that are used to copy objects, as shown in Example 1-64.

copy(object) => object creates a “shallow” copy of the given object. In this context, shallow means that the object itself is copied, but if the object is a container, the members will still refer to the original member objects.

Example 1-64. Using the copy Module to Copy Objects

File: copy-example-1.py

import copy

a = [[1],[2],[3]]
b = copy.copy(a)

print "before", "=>"
print a
print b

# modify original
a[0][0] = 0
a[1] = None

print "after", "=>"
print a
print b

before =>
[[1], [2], [3]]
[[1], [2], [3]]
after =>
[[0], None, [3]]
[[0], [2], [3]]

You can also make shallow copies of lists using the [:] syntax (full slice), and you can make copies of dictionaries using the copy method.

In contrast, deepcopy(object) => object creates a “deep” copy of the given object, as shown in Example 1-65. If the object is a container, all members are copied as well, recursively.

Example 1-65. Using the copy Module to Copy Collections

File: copy-example-2.py

import copy

a = [[1],[2],[3]]
b = copy.deepcopy(a)

print "before", "=>"
print a
print b

# modify original
a[0][0] = 0
a[1] = None

print "after", "=>"
print a
print b

before =>
[[1], [2], [3]]
[[1], [2], [3]]
after =>
[[0], None, [3]]
[[1], [2], [3]]