Converting List Comprehensions to For Loops in Python

Question

I understand the importance of list comprehensions, but do not understand their inner-workings, thus am not able to understand them in simpler terms such as I would a for lo

Answer 1

Your question seems to imply that this snippet is already inside of a for loop, like this:

outer_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

for index in xrange(3):
    li = [row[index] for row in outer_list]
    print li

which would output:

[1, 4, 7]
[2, 5, 8]
[3, 6, 9]

You could convert this to use only conventional for loops by nesting an inner loop inside of your outer loop:

outer_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

for index in xrange(3):
    li = []                    # initial empty list
    for row in outer_list:     # our inner loop
        li.append(row[index])  # Build the list "li" iteratively
    print li

If you are treating this nested list as a matrix, then what you describe is essentially taking the transpose of the matrix. You can do this entirely via list comprehensions as well:

outer_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

transpose = [[row[i] for row in outer_list] for i in range(len(outer_list[0]))]

print transpose

which yields:

[[1, 4, 7],
[2, 5, 8],
[3, 6, 9]]

As shown in the official Python tutorial here.

I've included the nested list comprehension example by way of completeness but a word to the wise: using nested list comprehensions generally makes code less comprehensible, not more. Just because you can doesn't mean you should.