Nested list comprehensions

"Python's for loop are for looping over an iterable while doing something. So when I see a loop within a loop, I think "nested looping". I don't immediately think "we're building up a list-of-lists". I'd need to read through most of this code to figure out that's its purpose. Comprehensions are for specifically for building up new lists. So whenever I see a comprehension, I think "we're build a new list"."

"This is a list comprehension with another list comprehension inside its mapping part. List comprehensions create a new list, and since we've embedded a comprehension within a comprehension, this code creates a list-of-lists: I don't find this code very readable: But I don't think the comprehensions are the problem here. The problem is our use of whitespace. Here's the same code broken up over multiple lines: I find this code much more readable."

Nested list comprehensions produce a list-of-lists by embedding one comprehension inside another. Readability depends heavily on whitespace and how the comprehension is laid out across lines. Breaking a nested comprehension over multiple lines can make intent and structure clearer. Nested for-loops emphasize iterative control flow rather than the data structure being built. Using a comprehension signals list construction, making the resulting data structure more apparent at a glance. Combining an inner comprehension with an outer loop can be clearer than deeply nested loops. Avoid overly complex comprehensions that obscure intent.

Read at Pythonmorsels