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/typ

Answer 1

You can use the typeguard module which can be installed with pip. It provides you with a function check_argument_types or a function decorator @typechecked. which should do your runtime type checking for you: https://github.com/agronholm/typeguard

from typing import Union
from typeguard import check_argument_types, typechecked

def check_and_do_stuff(a: Union[str, int]) -> None:
    check_argument_types() 
    # do stuff ...

@typechecked
def check_decorator(a: Union[str, int]) -> None:
    # do stuff ...

check_and_do_stuff("hello")
check_and_do_stuff(42)
check_and_do_stuff(3.14)  # raises TypeError

If you want to check a type of a single variable for a different reason, you can use typeguard's check_type function directly:

from typing import Union
from typeguard import check_type

MyType = Union[str, int]

check_type("arg", "string", MyType, None)  # OK
check_type("arg", 42, MyType, None)  # OK
check_type("arg", 3.5, MyType, None)  # raises TypeError

The "arg" and None arguments are unused in this example. Note that the check_type function is not documented as a public function of this module so its API may be subject to change.