Are list comprehensions fully evaluated before being used in the rest of the code?

Question

Consider the following code.

with open('filename.txt', 'r') as f:
    var = [element for element in f.readlines()][3]

This question concerns the internals of Python, rather than the result.

Does Python calculate all the elements of the indexes in the entire list of [element for element in f.readlines()], or does Python just calculate all of the elements until the third index?

If you want to be able to abort the computation once some specific item has been found, you should use a generator expression instead of a list comprehension. — jasonharper, Jun 02 '22 at 02:08
Worth noting that `f.readlines()` returns a list, so the list comprehension is actually redundant. — wjandrea, Jun 02 '22 at 02:25

score 3 · Accepted Answer · answered Jun 02 '22 at 02:08

It calculates all of them. You can verify with something like this:

>>> [(i, print(i)) for i in range(3)][1]
0
1
2
(1, None)

This isn't really "internal", because this is well-defined behaviour and list comprehensions can have side-effects (even if they shouldn't).

Are list comprehensions fully evaluated before being used in the rest of the code?

1 Answers1