Asserting Type Guards that works with mypy ?

[tushar5526]

I am not sure if this is the right place to ask this question, but I have been bugged with a simple typing problem of needing a behaviour where I can have something like an "asserting" TypeGuard that works with mypy.

def check_obj_of_type(
    obj: typing.Any, validation_type: typing.Type[T], raise_error: bool = False
) -> TypeGuard[T]:
    """
    Generic Type Guard, that checks for a type and raises Type Error
    """
    if isinstance(obj, validation_type):
        return True
    if raise_error:
        raise TypeError(f"Object should be {validation_type} not '{type(obj)}'")
    return False

If I want to use check_obj_of_type with say a function like

class SomeClass:
    ...
    def do_some_stuff() -> "SomeType":
        if check_obj_of_type(self.some_var, SomeType, raise_error=True):
           return "SomeType"
    ...

mypy will error out for the above code stating Missing return statement but in-fact the code will never reach the else clause.

For mypy to pass successfully, one would have to either #type ignore[return] or add a an assert in else clause. Is there any other better way to do this ?

[erictraut]

There is currently no support in the Python type system for a "type assert". Other languages like TypeScript have this sort of feature.

There was some discussion of adding a TypeAssert feature, but there wasn't sufficient interest to move to the PEP stage with this proposal.

If you're able to modify the code in question, you can change the "check" function to return the value if its type matches and raise an error if not.

from typing import Any, Literal, TypeVar, overload

T = TypeVar("T")

@overload
def check_obj_of_type(
    obj: Any, validation_type: type[T], raise_error: Literal[True]
) -> T:
    ...

@overload
def check_obj_of_type(
    obj: Any, validation_type: type[T], raise_error: bool = False
) -> Any:
    ...

def check_obj_of_type(
    obj: Any, validation_type: type[T], raise_error: bool = False
) -> T | Any:
    if isinstance(obj, validation_type):
        return obj
    if raise_error:
        raise TypeError(f"Object should be {validation_type} not '{type(obj)}'")
    return False

class SomeType:
    ...

class SomeClass:
    some_var: Any

    def do_some_stuff(self) -> SomeType:
        return check_obj_of_type(self.some_var, SomeType, raise_error=True)

[tushar5526]

Thanks @erictraut I will try some workarounds!