Python: Essential Reference, Third Edition

References and Copies

When a program makes an assignment such as a = b, a new reference to b is created. For immutable objects such as numbers and strings, this assignment effectively creates a copy of b. However, the behavior is quite different for mutable objects such as lists and dictionaries. For example:

b = [1,2,3,4]
a = b                # a is a reference to b
a[2] = -100          # Change an element in 'a'
print b              # Produces '[1, 2, -100, 4]'

Because a and b refer to the same object in this example, a change made to one of the variables is reflected in the other. To avoid this, you have to create a copy of an object rather than a new reference.

Two types of copy operations are applied to container objects such as lists and dictionaries: a shallow copy and a ...