blacklist_test.py

from typing import overload, Sequence, Union, NewType
from mypy_extensions import NoReturn

# by convention, underlying type should not be 'str'
ArgList = NewType('ArgList', Sequence[str])

@overload
def make_arglist(x: str) -> NoReturn: ...

@overload
def make_arglist(x: Sequence[str]) -> ArgList: ...

def make_arglist(x: Sequence[str]) -> Union[ArgList, None]:
    # Or maybe even omit the check altogether to minimize runtime cost, since
    # the typechecker ought to be able to do this check for us
    if isinstance(x, str):
        raise Exception()
    else:
        return ArgList(x)

def bar(x: ArgList) -> str:
    return " ".join(x)

def main() -> None:
    # I want this to typecheck when using lists, tuples, custom sequences...
    x = make_arglist(["hello", "world"])
    print(bar(x))

    # But want this to fail noisily
    # (mypy currently silently stops typechecking instead)
    y = make_arglist("hello")
    print(bar(y))
    
if __name__ == '__main__':
    main()