Is using one line loops, even nested loops always a good practice in Python? I see a lot of people just love "one-liners", but to me they're hard to read sometimes, especially if we're talking about nested loops.

Moreover most of nested loops I've seen so far exceed the recommended 79 characters per line.

So I'd like to know if "one-liners" offer anything extra apart being compact? Do they use less memory maybe?

Yes, they may be easily faster, since more code may be run in C (in explicit loop all of immediate steps has to be available to interpreter, in one-liner list comprehension it does not have to). There is also overhead of .append method call, method look-up etc. In list comprehension all of that is avoided:

import timeitdef f1():result = []for x in range(100):for y in range(100):result.append(x * y)return resultdef f2():return [x * y for y in range(100) for x in range(100)]print('loop: ', timeit.timeit(f1, number=1000))
print('oneliner:', timeit.timeit(f2, number=1000))

Results (Python 3):

loop:  1.2545137699926272
oneliner: 0.6745600730064325