python - Nested list comprehension with two lists

Question

I understand how the simple list comprehension works eg.:

[x*2 for x in range(5)] # returns [0,2,4,6,8]

and also I understand how the nested list comprehesion works:

w_list = ["i_have_a_doubt", "with_the","nested_lists_comprehensions"]

# returns the list of strings without underscore and capitalized
print [replaced.title() for replaced in [el.replace("_"," ")for el in w_list]]

so, when I tried do this

l1 = [100,200,300]
l2 = [0,1,2]
[x + y for x in l2 for y in l1 ]

I expected this:

[100,201,302]

but I got this:

[100,200,300,101,201,301,102,202,302]

so I got a better way solve the problem, which gave me what I want

[x + y for x,y in zip(l1,l2)]

but I didn't understood the return of 9 elements on the first code

Answer 1

The reason it has 9 numbers is because python treats

[x + y for x in l2 for y in l1 ]

similarly to

for x in l2:
    for y in l1:
       x + y

ie, it is a nested loop