understanding “item for item in list_a if …” PYTHON

Question

I\'ve seen the following code many times, and I know it\'s the solution to my problems, but I\'m really struggling to understand HOW it works. The code in particular is:

Answer 1

It might be clearer if you split it into three parts:

1. Take item;
2. From for item in list;
3. Where item not in list_b.

The reason the list comprehension syntax is like this is firstly because that reflects the expanded version:

for item in list: # 2.
    if item not in list_b: # 3.
        new_list.append(item) # 1.

and also because you don't necessarily want just item, for example:

new = [x ** 2 for x in old if not x % 2]

will create a new list with the squares of all even numbers in old.