It doesn't affect runtime at all (except when it does :P, but that's a bug). So there is a difference between what's valid in mypy, and what's valid in "real" P

It doesn't affect runtime at all (except when it does :P, but that's a bug). So there is a difference between what's valid in mypy, and what's valid in "real" Python. Typing does associate type with the names as well, which isn't necessary for legal Python (but is often cleaner). For example, mypy will yell at you if you do something like this: def add_week_to_day(d: str) -> str: # d arrives as an ISO string, like '2016-12-23' d = datetime.strptime(d, '%Y-%m-%d') d = d + timedelta(days=7) return d.strftime('%Y-%m-%d') Because d was original defined as a str, and now I'm re-using it as a datetime. This is of course perfectly valid Python, and will work at runtime. But mypy prefers you use a different name for the different types.