mypy reporting error in generic function on return statement

With the code below

def incr[T: int | None](value: T) -> T:
if value is None:
    return value
return value + 1

incr(None)
incr(1)

Running mypy gives error: main.py:4: error: Incompatible return value type (got "int", expected "T") [return-value] Found 1 error in 1 file (checked 1 source file)

Why is this happening? It appears to me that the line is returning correct type. Note: the goal of such type annotation was to show that the function returns None only when given None as input

Solution

T: int | None is a type variable with an upper bound. It indicates that if you pass any subtype of int | None, you will get that subtype back.

mypy is warning you that if you pass a subclass of int, the function behaves wrongly at runtime. This is due to the definition of builtins.int__add__, which returns int (rather than typing.Self):

import typing as t

def incr[T: int | None](value: T) -> T:
    if value is None:
        return value
    return value + 1  # error: Incompatible return value type (got "int", expected "T")  [return-value]

class MyInt(int):
    pass

>>> result: t.Final = incr(MyInt(3))
>>> if t.TYPE_CHECKING:
...    reveal_type(result)  # note: Revealed type is "MyInt"
>>> print(type(result))
<class 'int'>

The solution is to use constrained type variables, which behaves by up-casting the result to the appropriate constrained type:

def incr[T: (int, None)](value: T) -> T: ...

>>> if t.TYPE_CHECKING:
...    reveal_type(result)  # note: Revealed type is "builtins.int"