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

From the list comprehension documentation:

When a list comprehension is supplied, it consists of a single expression followed by at least one for clause and zero or more for or if clauses. In this case, the elements of the new list are those that would be produced by considering each of the for or if clauses a block, nesting from left to right, and evaluating the expression to produce a list element each time the innermost block is reached.

In other words, pretend that the for loops are nested. Reading from left to right your list comprehension can be nested as:

for elem in vec:
    for num in elem:
        num           # the *single expression* from the spec

where the list comprehension will use that last, innermost block as the values of the resulting list.