All good points... just to add, PyCharm has a type hinting system (which informed the 3.5 Type Hints design), which knows the expected types for stdlib function

All good points... just to add, PyCharm has a type hinting system (which informed the 3.5 Type Hints design), which knows the expected types for stdlib functions, and can be taught the types of user-defined function arguments. It'll then warn you if it thinks you are passing the wrong type of argument to a function. Obviously not infallible, but it catches time-wasters not infrequently.