I have type aliases of Unions like so:

Request = Union[WriteRequest, ReadRequest]

I'd like to be able to use "Request" in an isinstance check rather than having to duplicate the information in a tuple to pass to the check. Currently I get:

TypeError: Subscripted generics cannot be used with class and instance checks

Seems like Unions could be used here interchangeably with tuples since they mean the same thing in this context. Of course this would only work if the element types are being capable of being used in isinstance checks.

I think this already works unless you are using TypeVars::

    t = Union[str, int]

    isinstance(1, t)    # True
    isinstance('1', t)  # True

If you are using TypeVars, though, it cannot work:

    T = TypeVar('my_type')
    thing = Union[T, str]

    isinstance('hi', thing)  # Exception

This makes sense, because ``T`` is just a name, not related to an actual object.

Sorry if I'm missing something, because I did try the non-working example and my exception actually says, "issubclass() arg 2 must be a class, a tuple of classes, or a union." The exception makes sense, but its difference from yours might mean I'm doing something differently.

Ah, this was implemented for 3.10 as a part of PEP 604. https://www.python.org/dev/peps/pep-0604/#isinstance-and-issubclass.

Feel free to close.