How does this function work?

[Guest]

... return [x for x in range(3), 1, y for y in range(4), 4]
...

>>> f(3) [[0, 1, 2], [0, 1, 2], 1, 1, 3, 3]
>>>

I've been staring at it for 15 minutes and I'm no closer to understanding than when I started.

Why is this even legal Python syntax? What is going on that makes it return what it does?

Jp