contextlib.contextmanager allows `Iterator[T]` -- should probably be `Generator[T, None, None]`?

[asottile]

For instance, this script passes mypy:

import contextlib
from typing import Iterator

@contextlib.contextmanager
def f() -> Iterator[int]:
    return iter([1])


with f():
    pass


with f():
    raise TypeError('wat')

but fails at runtime (and not in the expected way):

$ python t.py 
Traceback (most recent call last):
  File "t.py", line 14, in <module>
    raise TypeError('wat')
TypeError: wat

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "t.py", line 14, in <module>
    raise TypeError('wat')
  File "/usr/lib/python3.6/contextlib.py", line 99, in __exit__
    self.gen.throw(type, value, traceback)
AttributeError: 'list_iterator' object has no attribute 'throw'

[srittau]

Sounds reasonable.

[graingert]

contextlib.contextmanager doesn't need to be as specific as Generator[T, None, None], it could take the slightly wider type:

class ContextManagerArgReturnType(Protocol, Generic[T]):
    def __next__(self) -> T: ...
    def throw(self, exctype: Optional[Type[BaseException]], excinst: Optional[BaseException], exctb: Optional[TracebackType]) -> Any: ...

ContextManagerArgType = Callable[[], ContextManagerArgReturnType[T]]

[asottile]

this (unfortunately) caused a production incident today -- any chance this can be reconsidered (despite the backward incompatibility)?

[HacKanCuBa]

So, this is an interesting pitfall of using Iterator[] instead of Generator[] as the docs suggest:

A generator can be annotated by the generic type Generator[YieldType, SendType, ReturnType]
...
Alternatively, annotate your generator as having a return type of either Iterable[YieldType] or Iterator[YieldType]

I'm just bumping this up.

[AlexWaygood]

So, this is an interesting pitfall of using Iterator[] instead of Generator[] as the docs suggest:

A generator can be annotated by the generic type Generator[YieldType, SendType, ReturnType]
...
Alternatively, annotate your generator as having a return type of either Iterable[YieldType] or Iterator[YieldType]

I'm just bumping this up.

Can I ask why you're bumping it up? Did the current stubs mean that mypy failed to spot a bug in your code, the same as happened to @asottile?

[HacKanCuBa]

So, this is an interesting pitfall of using Iterator[] instead of Generator[] as the docs suggest:

A generator can be annotated by the generic type Generator[YieldType, SendType, ReturnType]
...
Alternatively, annotate your generator as having a return type of either Iterable[YieldType] or Iterator[YieldType]

I'm just bumping this up.

Can I ask why you're bumping it up? Did the current stubs mean that mypy failed to spot a bug in your code, the same as happened to @asottile?

Which brought me here after reading a blogpost :D