PEP 604: Add or operator to class type

[mano7onam]

Python 3.10 allowing writing union types as X | Y so class type now should have operator or

[jakebailey]

Does this need to be generic in some way to annotate that the return type is a Union, or is that a detail that does not matter for type checkers as they special case it?

e.g. (spitballing, don't take this as correct without someone else to vet me...):

_TT = TypeVar("_TT", bound="type")
_TT2 = TypeVar("_TT2", bound="type")

def __or__(self: _TT, t: _TT2) -> Union[_TT, _TT2]: ...

Note that no type checker runs for 3.10 as far as I'm aware, so there isn't much visibility into breakages caused by adding 3.10-related code. I sent #5153 to at least configure the CI checks to run pyright on 3.10.

[mano7onam]

@jakebailey or can be applied to the same type i.e. int | int so the result will be int

[jakebailey]

Sure, my example above isn't complex enough to capture that (I was spitballing), but could have an overload (guessing again):

def __or__(self: _TT, t: _TT) -> _TT: ...

I'm not questioning the construction of it, just whether it needs to be there to not break type checkers.

[east825]

To me, the generic version of the signature looks a bit too complicated, and I'm pretty sure that all type checkers will just special case it similarly to __class_getitem__. Besides, notice that the argument of __or__ is supposed to be a valid type hint, and the current type system doesn't really allow to express that (at least until TypeForm emerges). For instance, it will likely prohibit something like int | Callable[..., Any], because Callable is not bound to type (is it?). Also, since __or__ is defined on type itself, something trivial like int | str will give you Union[Type[int], Type[str]], which is not what it really describes.

I would keep it as simple as the following and let type checkers work out all the magic interpreting the syntax :)

def __or__(self, t: Any) -> Any: ...

[JelleZijlstra]

Suggested change

	def __or__(self, t: Any) -> types.Union: ...
	def __or__(self, t: Any) -> Any: ...

As you mentioned, int | int returns int, not a Union. We could do something more clever with overloads, but I'm not sure that's worth it since type checkers will surely special-case this.

[mano7onam]

The type of int | int will still be types.Union
so we can leave it as it is even for such cases.

[JelleZijlstra]

Ah, I was confused because it gets printed as int, but it's indeed a Union object at runtime. Sorry for the confusion.

Jelles-Air:cpython jelle$ ./python.exe 
Python 3.10.0a7+ (heads/master:28d28e053d, Apr  8 2021, 07:40:29) [Clang 11.0.3 (clang-1103.0.32.59)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> int | int
int
>>> type(_)
<class 'types.Union'>