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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 deep copy. A shallow copy creates a new object, but populates it with references to the items contained in the original object. For example:

b = [ 1, 2, [3,4] ]
a = b[:]          # Create a shallow copy of b.
a.append(100)     # Append element to a.
print b           # Produces '[1,2, [3,4]]'. b unchanged.
a[2][0] = -100    # Modify an element of a.
print b           # Produces '[1,2, [-100,4]]'.

In this case, a and b are separate list objects, but the elements they contain are shared. Therefore, a modification to one of the elements of a also modifies an element of b, as shown.

A deep copy creates a new object and recursively copies all the objects it contains. There is no built-in function to create deep copies of objects. However, the copy.deepcopy() function in the standard library can be used, as shown in the following example:

import copy
b = [1, 2, [3, 4] ]
a = copy.deepcopy(b)
a[2] = -100
print a  # produces [1,2, -100, 4]
print b  # produces [1,2,3,4]
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