[Python] How to check a variable against Union type during runtime?

I'm trying to write a function decorator that uses Python 3.6's type hints to check that a dictionary of arguments respects the type hints and if not raise an error with a clear description of the problem, to be used for HTTP APIs.

The problem is that when the function has a parameter using the Union type I can't check a variable against it at runtime.

For example, I have this function

from typing import Union
def bark(myname: str, descr: Union[int, str], mynum: int = 3) -> str:
return descr + myname * mynum


I can do:

isinstance('Arnold', bark.__annotations__['myname'])


But not:

isinstance(3, bark.__annotations__['descr'])


Because Union cannot be used with isinstance or issubclass.

I couldn't find a way to check it using the type object. I tried to implement the check by myself but while bark.__annotations__['descr'] is shown as typing.Union[int, str] in the REPL I can't access the list of the types at runtime, if not using the ugly hack of examining bark.__annotations__['descr'].__repr__().

Is there a proper way to access this information? Or is it deliberately intended to not be easily accessible at runtime?

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