List Comprehensions: A First Look

Now that we’ve seen how the iteration protocol works, let’s turn to a very common use case. Together with for loops, list comprehensions are one of the most prominent contexts in which the iteration protocol is applied.

In the previous chapter, we learned how to use range to change a list as we step across it:

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>>> L = [1, 2, 3, 4, 5]

>>> for i in range(len(L)):
...     L[i] += 10
...
>>> L
[11, 12, 13, 14, 15]

This works, but as I mentioned there, it may not be the optimal “best-practice” approach in Python. Today, the list comprehension expression makes many such prior use cases obsolete. Here, for example, we can replace the loop with a single expression that produces the desired result list:

>>> L = [x + 10 for x in L]
>>> L
[21, 22, 23, 24, 25]

The net result is the same, but it requires less coding on our part and is likely to run substantially faster. The list comprehension isn’t exactly the same as the for loop statement version because it makes a new list object (which might matter if there are multiple references to the original list), but it’s close enough for most applications and is a common and convenient enough approach to merit a closer look here.