Unclear python syntax

Question

I'm having a hard time understanding the following list comprehension:

words = [word for line in open('words.txt', 'r') for word in line.split()]

I understand the first part, the word for line in open('words.txt', 'r') but the second part confuses me. Does it mean for each line in words.txt, loop for each word on the split line?

@JeanFrançoisFabre: that dupe is for using a list comprehension as the expression of another list comprehension, not for using multiple for loops in a single list comprehension. — Martijn Pieters, May 08 '17 at 19:26
The second answer there may cover this case, but the answer is in entirely the wrong context there (it is the wrong answer for the question posted). — Martijn Pieters, May 08 '17 at 19:28
I note that the [Python tutorial](https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions) covers the syntax in enough detail, including multiple `for` loops. — Martijn Pieters, May 08 '17 at 19:38
@MartijnPieters so anyone can post any list comprehension and gets a personalized explanation? — Jean-François Fabre, May 08 '17 at 19:40
@Jean-FrançoisFabre: no, we just had to find a better dupe. — Martijn Pieters, May 08 '17 at 19:59

Martijn Pieters · Accepted Answer · 2017-05-08T19:25:11.613

The for loops are nested, from left to right. You can write it out as regular loops like this:

words = []
for line in open('words.txt', 'r'):
    for word in line.split():
        words.append(word)

So the expression before the for loops is the final value added to the produced list, and all the for loops (and any if tests) are executed nested inside the preceding loop or test.

So, yes, for each line in the opened file, the line is split and the resulting words from those lines are added to the list being built.

Unclear python syntax

1 Answers1