None vs falsy - index at 0

Question

Accepted Answer

The target 1 is at index 0, so first_index returns 0. The problem is that 0 is falsy in Python, so 'if index' evaluates to False even though the item was found. The correct check is 'if index is not None', which distinguishes a valid result of 0 from an actual absence of the item. This is one of the most common bugs in Python search functions.