I was just thinking that squaring an imaginary number results in a real number - that's a pretty crazy thing for a type system to handle, no? I guess that means

I was just thinking that squaring an imaginary number results in a real number - that's a pretty crazy thing for a type system to handle, no? I guess that means the correct annotations would be more along the lines of: def square(number: numbers.Complex) -> numbers.Real: ... T = typing.TypeVar('T', numbers.Real) def square(number: T) -> T: return number ** 2 What a rabbit hole!