types.GenericAlias vs typing._GenericAlias

Python Help
1

Hi, today I found a surprising fact that types.GenericAlias and typing._GenericAlias are different. Seems user defined generic uses typing._GenericAlias and builtin types uses types.GenericAlias

>>> type(list[int])
<class 'types.GenericAlias'>
>>> class Foo[T]: pass
... 
>>> type(Foo[int])
<class 'typing._GenericAlias'>
>>> type(list[int]) is type(Foo[int])
False

Is this intentional or some sort of bug? What is the correct way to check if an object is an generic alias?

My Python version is 3.12.3

2

It’s intentional, there’s some relevant documentation in the typing source:

3

Thanks for the context!

If typing._GenericAlias is not documented nor public, does that mean currently there is no reliable and future-proof way to check an GenericAlias?

4

What’s goal of knowing that? What makes Foo[int] and list[int] related?

If your goal is whether type has type arguments/generic base you can use get_origin/get_args. If get_origin returns None then it’s not a subscripted generic type.

5

I am working on a stub generator, that involves evaluating generic types (replace TypeVar with concrete types). In this case builtin types are not very different from user-defined generics.

get_origin will do the job, but it not as readable or type-checker-friendly as a match statement in my case like:

match t:
  case TypeAliasTypes():
    ...
  case GenericAlias():
    ...