[issue29302] add contextlib.AsyncExitStack

[Nick Coghlan]

Nick Coghlan added the comment:

While it *may* be possible to do something simpler for test purposes where performance isn't a major concern, fully supporting type() level mocking basically requires bringing the equivalent of wrapt object proxies into the standard library: https://wrapt.readthedocs.io/en/latest/wrappers.html#object-proxy

I actually think we *should* do that, and occasionally bug Graham Dumpleton about it, but while he's not opposed to the idea, it's also something that would take quite a bit of work (since odd edge cases that are tolerable in an opt-in third party module would probably need to be addressed for a standard library version).

For test cases like AsyncExitStack though, we instead just use custom type definitions, rather than the unittest.mock module.

Autospec'ed mocks are most attractive when we're dealing with object interfaces that are subject to a high rate of churn (since they make the tests more self-adjusting), and that isn't the case here: Python's syntactic support protocols rarely change, and when they do, preserving backwards compatibility with existing classes is typically a key requirement.

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<http://bugs.python.org/issue29302>
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