Typing __exit__ in 3.5 fails on runtime, but typechecks

Question

What is the proper type signature for __exit__? I have the following:

from types import TracebackType
from typing import Optional, Type

class Foo:
    def __enter__(self) -> 'Foo':
        return self

    def __exit__(self, exc_type: Optional[Type[BaseException]],
                 exc_value: Optional[BaseException],
                 traceback: Optional[TracebackType]) -> bool:
        return False

On recent mypy (0.560) this typechecks with --strict (and I have moderate faith in this signature, because I stole it from the innards of the typeshed).

When this script is run with python 3.6, as expected nothing happens. But when run with 3.5.2, we get an exception:

Traceback (most recent call last):
  File "/home/student/mypy_test/test.py", line 4, in <module>
    class Foo: #(ContextManager['Foo']):
  File "/home/student/mypy_test/test.py", line 8, in Foo
    def __exit__(self, exc_type: Optional[Type[BaseException]],
  File "/usr/lib/python3.5/typing.py", line 649, in __getitem__
    return Union[arg, type(None)]
  File "/usr/lib/python3.5/typing.py", line 552, in __getitem__
    dict(self.__dict__), parameters, _root=True)
  File "/usr/lib/python3.5/typing.py", line 512, in __new__
    for t2 in all_params - {t1} if not isinstance(t2, TypeVar)):
  File "/usr/lib/python3.5/typing.py", line 512, in <genexpr>
    for t2 in all_params - {t1} if not isinstance(t2, TypeVar)):
  File "/usr/lib/python3.5/typing.py", line 1077, in __subclasscheck__
    if super().__subclasscheck__(cls):
  File "/home/student/.local/share/virtualenvs/sf_cs328-crowdsourced-QAuuIxFA/lib/python3.5/abc.py", line 225, in __subclasscheck__
    for scls in cls.__subclasses__():
TypeError: descriptor '__subclasses__' of 'type' object needs an argument

If you remove parameters until the exception disappears, we find that the problem type is the first: exc_type: Optional[Type[BaseException]].

Note: To get it to complain if the type signature does not match (when run with mypy), you need to change class Foo: to class Foo(ContextManager['Foo']). I didn't do this in the snippet, because typing in Python 3.5.2 is missing Coroutine, Awaitable, ContextManager, etc (this is the version in LTS releases of old-ish distros). I wrote about a workaround for this here: https://stackoverflow.com/a/49952293/568785. So I guess, the full reproducible example is:

# Workaround for ContextManager missing in 3.5.2 typing
from typing import Any, TypeVar, TYPE_CHECKING
try:
    from typing import ContextManager
except ImportError:
    class _ContextManager:
        def __getitem__(self, index: Any) -> None:
            return type(object())

    if not TYPE_CHECKING:
        ContextManager = _ContextManager()

# The actual issue:
from types import TracebackType
from typing import Optional, Type

class Foo:
    def __enter__(self) -> 'Foo':
        return self

    def __exit__(self, exc_type: Optional[Type[BaseException]],
                 exc_value: Optional[BaseException],
                 traceback: Optional[TracebackType]) -> bool:
        return False

I've verified that not inheriting ContextManager for running in Python 3.5.2 still produces the error (so this exception isn't a product of this hack, it's a product of the 3.5.2 runtime's typing library not liking the signature of __exit__).

Presumably, this is another bug in Python 3.5's typing library. Is there a sane way to work around this?

Answer 1

A ugly workaround that I'm using for now, is use TYPE_CHECKING to determine if I should just fake the type:

from typing import Type, TYPE_CHECKING

if TYPE_CHECKING:
    BaseExceptionType = Type[BaseException]
else:
    BaseExceptionType = bool # don't care, as long is it doesn't error

Then you can do:

def __exit__(self, exc_type: Optional[BaseExceptionType],
             exc_value: Optional[BaseException],
             traceback: Optional[TracebackType]) -> bool:
    return False

Verified against Python 3.5.2 and mypy 0.560.

This of course breaks any RTTI, but AFAIK RTTI is part of a PEP that didn't land (experimentally) until 3.6 or 3.7. This does break typing.get_type_hints() though, obviously.