The way that an iterator signals that it is done iterating is by raising StopIteration ( https://docs.python.org/3/library/exceptions.html#StopIteration https:/

The way that an iterator signals that it is done iterating is by raising StopIteration ( https://docs.python.org/3/library/exceptions.html#StopIteration https://docs.python.org/3/library/exceptions.html#StopIterat... ). I don't know if it's still the case in Python 3, but in 2 that exception was core to how for loops and list comprehensions worked.