Explanation of how nested list comprehension works?

Question

I have no problem for understanding this:

a = [1,2,3,4]
b = [x for x in a]

I thought that was all, but then I found this snippet:

Answer 1

Yes, you can nest for loops INSIDE of a list comprehension. You can even nest if statements in there.

dice_rolls = []
for roll1 in range(1,7):
    for roll2 in range(1,7):
        for roll3 in range(1,7):
            dice_rolls.append((roll1, roll2, roll3))

# becomes

dice_rolls = [(roll1, roll2, roll3) for roll1 in range(1, 7) for roll2 in range(1, 7) 
              for roll3 in range(1, 7)]

I wrote a short article on medium explaining list comprehensions and some other cool things you can do with python, you should have a look if you're interested : )