Confusing behavior with raise Class

[bwo]

from typing import Type, Mapping, TypeVar


class Foo(Exception):
    pass


class Bar(Foo):
    pass


m = {1: Bar, 2: Foo}  # Mapping[int, Type[Foo]]


def f(i):
    # type: (int) -> Type[Foo]
    return m[i]


def x():
    # type: () -> None
    reveal_type(f(1))
    raise f(1)


def y():
    # type: () -> None
    reveal_type(m[1])
    raise m[1]


# for completeness's sake
def huh(*args, **kwargs):
    # type: (*object, **object) -> Foo
    return Bar()


def z():
    # type: () -> None
    reveal_type(huh)
    raise huh

The first reveal_type shows that f(1) returns a Type[Foo]; x() does not typecheck, with the error "Exception must be derived from BaseException".

The second reveal_type shows that m[1] is def (*args: builtins.object, **kwargs: builtins.object) -> foo.Foo. y() typechecks.

The third reveal_type shows that huh is def (*args: builtins.object, **kwargs: builtins.object) -> foo.Foo. z() does not typecheck (same error as x()).

[gvanrossum]

Could you describe in English what you think is wrong and what you would expect instead? It's a rather long example to try and understand.

[JelleZijlstra]

Here's a really short repro:

$ python3.6 -m mypy -c 'from typing import Type; X: Type[Exception]; raise X'
<string>:1: error: Exception must be derived from BaseException

mypy -c 'raise Exception' works fine though.

mypy seems to not realize that Type[Exception] is compatible with raise.

[bwo]

@gvanrossum sorry, though @JelleZijlstra has basically got it.

If I have a Type[Exception] (as returned by f), I ought to be able to raise it, but instead, mypy complains.

mypy does not complain if I just write raise Bar, but it also thinks (according to reveal_type) that Bar is a function type returning a an instance of Bar. It will, however, complain if I type raise huh, even though reveal_type(huh) and reveal_type(Bar) have identical output.

[gvanrossum]

Ah, this is definitely a bug. Do you want to give fixing it a try?

[bwo]

Sure, but I could do with a pointer as to where to start.

[Michael0x2a]

@bwo -- A good starting point is often to grep for the error message -- you can then see where it's called, attach a breakpoint, etc.

To save you a little bit of work, you'll probably want to start with the type_check_raise method in mypy/checker.py.