mysum.py:9: error: Incompatible types in assignment ↪(expression has type "float", variable has type "int") What's the problem? Simply put, when I create output as a variable, I'm giving it an integer ...

Mypy will complain: mytest.py:13: error: Incompatible types in assignment ↪(expression has type "str", variable has type ↪"Optional[int]") Sure enough, you now can assign either None or an integer to ...

For some time now, Python developers have been able to “annotate” names with type information. With Python 3.5, type hints officially became part of the language (see PEP 484).