10.19. Pure Functions

A pure function does not produce side effects. It communicates with the calling program only through parameters (which it does not modify) and a return value. Here is the doubleStuff function from the previous section written as a pure function. To use the pure function version of double_stuff to modify things, you would assign the return value back to things.

 
1
def doubleStuff(a_list):
2
    """ Return a new list in which contains doubles of the elements in a_list. """
3
    new_list = []
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    for value in a_list:
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        new_elem = 2 * value
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        new_list.append(new_elem)
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    return new_list
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​
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things = [2, 5, 9]
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print(things)
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things = doubleStuff(things)
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print(things)
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​

(ch09_mod2)

Once again, codelens helps us to see the actual references and objects as they are passed and returned.

Python 2.7
1def doubleStuff(a_list):
2    """ Return a new list in which contains doubles of the elements in a_list. """
3    new_list = []
4    for value in a_list:
5        new_elem = 2 * value
6        new_list.append(new_elem)
7    return new_list
8
9things = [2, 5, 9]
10things = doubleStuff(things)
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(ch09_mod3)

Next Section - 10.20. Which is Better?