Unsupported operand types for + ("Union[int, float]" and "int")

[wsanchez]

mypy is complaining about math.py thusly:

src/sample_klein_app/application/math.py: note: In member "add" of class "Application":
src/sample_klein_app/application/math.py:54: error: Unsupported operand types for + ("Union[int, float]" and "int")
src/sample_klein_app/application/math.py: note: In member "subtract" of class "Application":
src/sample_klein_app/application/math.py:70: error: Unsupported operand types for - ("Union[int, float]" and "int")
src/sample_klein_app/application/math.py: note: In member "multiply" of class "Application":
src/sample_klein_app/application/math.py:85: error: Unsupported operand types for * ("Union[int, float]" and "int")
src/sample_klein_app/application/math.py: note: In member "divide" of class "Application":
src/sample_klein_app/application/math.py:101: error: Unsupported operand types for / ("Union[int, float]" and "int")

[wsanchez]

What's going on, I think, is that we have:

def numberify(string: str) -> Union[int, float]:
    ...

And the math methods are doing something like:

self.numberify(a) + self.numberify(b)

And mypy thinks that the arithmetic operators don't like Union[int, float] and int, even though it's totally OK to operate on float and int combinations:

>>> 1.0 + 1
2.0

A confusing aspect for me is that int is part of the complaint at all; this is actually a Union[int, float]s being added to another Union[int, float], not an int.

[wsanchez]

Looks like a mypy bug: python/mypy#2128