Variable scope in Python nested function [duplicate]

Answer 1

It seems this is a commonly asked question as stated in this link. The reason is that variable a inside swim becomes a local variable as soon as there is an assignment to a. It shadows the external a, and local a is not defined before assignment in function swim, so the error rises.

Thanks for all your guys' answers!

Answer 2

You are appending an element in the first code. The id of a is still same.

But at second code, you are re-defining variable a, which is changing the id of that variable. So that you get UnboundLocalError.

Answer 3

When you do such assignment such as a = [1] + a or a += [1] in a scope, the variable would become local to that scope. In your case, that is function swim().