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Mock doesn't support stuff like __aexit__ which means you can't mock out something like an async context manager.

I'm interested in taking this on. I'm considering for this approach, creating a new MagicProxy class specifically for async, and assigning __aexit__ and __aenter__ to those.
The __call__ method itself has to be detected as asynchronous, and I don't believe it's possible for a __call__ to behave appropriately for both asynchronous and synchronous cases.

Taking a look at the code, this may require a bit more discussion.
Mock's classes create copies of themselves when an attribute is accessed. As such, I can't just add __aexit__ and have it generate a different mock type entirely, unless I know exactly what an async variant of this mock is.
If there was a way to specifically provide a function for asynchronous situations, like __acall__, that would make this a lot easier as well.
Anyway, my proposal now is:
create new classes MockAsync and MagicMockAsync, and have MockAsync and MagicMockAsync implement the __aexit__ and __aenter__ methods.
How is that as an approach?

There's also __aiter__, but it's semantics might be changing (issue #27243).
Robert or Michael, any opinions on Yusuke's proposal?

I found this while trying to test an async context manager. This is a critical feature to enable migrations to async code, as the impossibility to test something properly is not acceptable in many environments.
Implementing it in a way that __call__ returns an object capable of being coroutine or normal function would avoid having to implement Async specific Mocks, wouldn't it? I am not too confident, but would it be doable to have an implementation that depends on whether _is_coroutine is accessed or not?
I don't like it, but I really don't like the fact that we need to patch different all the methods depending on whether they are coroutine or not.
Obviously, having the __acall__ method would really help solving this issue.

I guess that it's worth noting that the same problem is being talked about in an issue on the asynctest library: https://github.com/Martiusweb/asynctest/issues/29 

Perhaps it's possible to vendor asynctest mock in the same vein as `mock` found it's way into unittest?
The real power of `asynctest` is in constructs like:
with asynctest.mock.patch("module.Class", autospec=True):
 ...
Where mock instances know what methods are async.

New changeset 77b3b7701a34ecf6316469e05b79bb91de2addfa by Lisa Roach in branch 'master':
bpo-26467: Adds AsyncMock for asyncio Mock library support (GH-9296)
https://github.com/python/cpython/commit/77b3b7701a34ecf6316469e05b79bb91de2addfa

Added and AsyncMock class which supports mocking async magic methods.

Adding Lisa to potentially add the PR from Ilya.

New changeset d6d6e2aa0249bb661541705335ddbb97a53d64c8 by Benjamin Peterson (Ilya Kulakov) in branch 'master':
Add Ilya Kulakov to Misc/ACKS. (GH-17130)
https://github.com/python/cpython/commit/d6d6e2aa0249bb661541705335ddbb97a53d64c8

New changeset e5125f7b3b88d8d4814ed7bddbd4f34d24d763dd by Miss Islington (bot) in branch '3.8':
Add Ilya Kulakov to Misc/ACKS. (GH-17130)
https://github.com/python/cpython/commit/e5125f7b3b88d8d4814ed7bddbd4f34d24d763dd