Check a variable against Union type at runtime in Python 3.6

Question

UPDATE (September 2020): Python 3.9 includes the typing.get_type_hints function for this use case, see https://docs.python.org/3.9/library/typing.html#typing.get_type_hints

I'm trying to write a function decorator that uses Python 3.6 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?

Answer 1

The existing accepted answer by MSeifert (https://stackoverflow.com/a/45959000/7433423) does not distinguish Unions from other generic types, and it is difficult to determine at runtime whether a type annotation is a Union or some other generic type like Mapping due to the behavior of isinstance() and issubclass() on parameterized Union types.

It appears that generic types will have an undocumented __origin__ attribute which will contain a reference to the original generic type used to create it. Once you have confirmed that the type annotation is a parameterized Union, you can then use the also undocumented __args__ attribute to get the type parameters.

>>> from typing import Union
>>> type_anno = Union[int, str]
>>> type_anno.__origin__ is Union
True
>>> isinstance(3, type_anno.__args__)
True
>>> isinstance('a', type_anno.__args__)
True