Is there a way to be generic over the parameters of a generic type?

[fjarri]

For example, I want a method to accept a callable that takes a Union type with any parameters and returns a value of that Union type (or, in other words, a value of any type from the union). That is, something like

Callable[Type[Union[*P]], Union[*P]]

For example, if it is given an argument Union[int, str] (the type, not a value of this type), it will return a value of type int or of type str.

ParamSpec and TypeVar doesn't seem to work here. Is it currently possible in Python?

A practical application here is a deserializing hook: it is given a type (possibly a generic, like Union of something), and a value of type Any, and it tries to deserialize it into that type.

[TeamSpen210]

Unfortunately no. First, something like Union[...] isn't valid to pass to a function accepting type - that only accepts actual classes. The solution to that is likely a new typing construct - TypeForm. The PEP for that is still in draft stage though. Secondly Union[*Ts] isn't supported itself. This was mentioned in early versions of the TypeVarTuple PEP, but was removed due to the limited number of use cases and implementation complexity required.