python list comprehension double for

Question

vec = [[1,2,3], [4,5,6], [7,8,9]]
print [num for elem in vec for num in elem]      <----- this

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

This is

Answer 1

You can look at list comprehension just as sequential statements. This applies for any levels of for and if statements.

For example, consider double for loop with their own ifs:

vec = [[1,2,3], [4,5,6], [7,8,9]]
result = [i for e in vec if len(e)==3 for i in e if i%2==0]

Here the list comprehension is same as:

result = []
for e in vec: 
    if len(e)==3:
        for i in e:
            if i%2==0:
                result.append(i)

As you can see list comprehension is simply for and if without indentations but in same sequence.