Source: 📖 Effective Python item 28

Multiple looping in comprehensions

You can loop multiple times when using list comprehensions to access deeper nested elements. These sub expressions run in the order provided, from left to right.

matrix = 1, 2, 3], [4, 5, 6], [7, 8, 9
flat = [x for x in row for row in matrix]
print(flat)

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

Including more than two loops in a comprehension becomes too visually noisy and difficult to follow, and it isn't much shorter than fully fleshed out for loops anyway. In this case, it's best to use a full for loop nesting sequence, in which the indentations will make it far easier to read.

See also:

• Comprehensions can affect and filter items in a list more clearly than map and filter