List Repetition in Python

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Today we’ll learn how repetition works with lists.

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You can use the * operator to repeat the elements of a list:

>>> letters = ["n", "q", "w"]
>>> letters * 3
['n', 'q', 'w', 'n', 'q', 'w', 'n', 'q', 'w']

You can also apply the repetition operator to nested lists. Here’s a list called codes:

>>> codes = [17, 5, 41]

The list codes becomes an element of a larger list. The element of the outer list, which is the inner list, is then repeated 4 times:

>>> triplets = [codes] * 4  
>>> triplets
[[17, 5, 41], [17, 5, 41], [17, 5, 41], [17, 5, 41]]

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Now let’s change the second element in the first inner list from 5 to 30:

triplets[0][1] = 30

Turns out the element was changed in each of the inner lists. This is because the repetition operator creates four references to the list codes:

>>> triplets
[[17, 30, 41], [17, 30, 41], [17, 30, 41], [17, 30, 41]]

This is because of shared references. If you want to learn more about them, you can read this article.

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