two elements at a time in list comprehension

>>> f1 = lambda x: x
>>> f2 = lambda x: 10*x

>>> [f(x) for x in range(5) for f in (f1,f2)]
[0, 0, 1, 10, 2, 20, 3, 30, 4, 40]

Here is what the above code is Doing:
1. The first for loop is executed first.
2. The second for loop is executed second.
3. The list comprehension is executed third.