constrain a generic type with an alias derived from a union type in Python

[GENDRAUD]

I know to constraint a generic being a type in a list. See the example

from typing import TypeVar

T = TypeVar("T", int, str)

def combine2(a: T, b: T) -> T:
    return  a + b

r: int = combine2(2, 2)
print(r)

s: str = combine2("Hello, ", "World!")
print(s)

If I want to do the same with the python 3.13 generic syntax, I write this code:

def combine2[T: (int , str)](a: T, b: T) -> T:
    return a + b

r: int = combine2(2, 2)
print(r)

s: str = combine2("Hello, ", "World!")
print(s)

It works but if I have several function taking a generic constraint with the same 2 type. I have to write every time the same T: (int , str)
It is not good. For instance If to change the constraint, I have to change it everywhere.

Therefore what I propose is a way to say that type that T can take comes from a Union type. If the constraint change. It has thefollowing advantage. If I have to change the constraint I only have to change the union type.

concrete example of what I would like to have

type constraint= int | str

def combine2[T from constraint](a: T, b: T) -> T:
    return a + b

r: int = combine2(2, 2)
print(r)

s: str = combine2("Hello, ", "World!")
print(s)

I don t think this feature exist. I already ask. There (https://stackoverflow.com/questions/79126319/how-to-constrain-a-generic-type-with-an-alias-derived-from-a-union-type-in-pytho) for instance

[sterliakov]

This isn't something mypy can introduce as a type checker - you're proposing either a python syntax change or at least a new typing core feature. Consider asking on Python's Discourse (https://discuss.python.org/c/typing/32) instead.