TypeError: 'builtin_function_or_method' object is not subscriptable

You are trying to access pop as if was a list or a tupple, but pop is not. It's a method.

Looks like you typed brackets instead of parenthesis by mistake.

instead of writing listb.pop[0] write

listb.pop()[0]
         ^
         |

I think you want

listb.pop()[0]

The expression listb.pop is a valid python expression which results in a reference to the pop method, but doesn't actually call that method. You need to add the open and close parentheses to call the method.