I would like to create a typing special form that takes a non-type, say integer, as an argument, similar to the 2nd argument of typing.Annotated.

For example, I’d like foo[5] to be the same type annotation as Annotated[int, 5].

There are many ways that don’t fail at runtime, but are rejected by the type checker. I’m using mypy 1.11.1 – latest version at this time.

import typing

class foo:
   def __class_getitem__(cls, param: int):
      return typing.Annotated[int, param]

foo5 : foo[5]

mypy doesn’t like it:

"foo" expects no type arguments, but 1 given  [type-arg]
Invalid type: try using Literal[5] instead?  [valid-type]

Using _SpecialForm:

@typing._SpecialForm
def bar(_self, param : int):
    return typing.Annotated[int, param]

bar5 : bar[5]

Nope:

Too many arguments for "_SpecialForm"  [call-arg]
Function "ex_typing_1.bar" is not valid as a type  [valid-type]
Perhaps you need "Callable[...]" or a callback protocol?
Invalid type: try using Literal[5] instead?  [valid-type]

Even the parenthesized syntax I don’t care much for is no-go:

def baz(param : int):
    return typing.Annotated[int, param]

baz5 : baz(5)

Sez mypy:

Invalid type comment or annotation  [valid-type]
Suggestion: use baz[...] instead of baz(...)