Improve type hints for attrs classes

[DMRobertson]

An indulgent sorbet.

Commitwise reviewable.

In particular, according to python-attrs/attrs#313 we should now be able to mark attrs classes using slots=True as Generic, as our minimum version of Python is 3.8. (Though let's confirm that in CI...)

[clokep]

Looks reasonable overall, but a few qustions!

[clokep]

Not sure it matters, but is the yield less efficient?

I'm not 100% sure I understand removing __iter__ and __next__.

[DMRobertson]

Err, being honest it's because I couldn't work out how to annotate the __iter__ and __next__ in a way that mypy was happy with. Can dig up the errors if you'd like.

I think this is equivalent though:

• __await__ is supposed to return an iterator
• an iterator is a thing with a __next__
• (so was __iter__ redundant? Might have needed it before the await syntax added in Py 3.5?)

[clokep]

Oh, I think it wasn't redundant because we were returning ourself, now __await__ itself is an iterator?

[DMRobertson]

Oh, I think it wasn't redundant because we were returning ourself, now __await__ itself is an iterator?

This explains why __next__ was needed before my change. But I assert that __iter__ is redundant these days:

$ python3.8 -m asyncio
asyncio REPL 3.8.17 (default, Jun  8 2023, 00:00:00) 
[GCC 13.1.1 20230511 (Red Hat 13.1.1-2)] on linux
Use "await" directly instead of "asyncio.run()".
Type "help", "copyright", "credits" or "license" for more information.
>>> import asyncio
>>> class DummyAwaitable:
...     def __init__(self, value):
...         self.value = value
...     
...     def __await__(self):
...         return self
...     
...     def __next__(self):
...         raise StopIteration(self.value)
... 
>>> 
>>> await DummyAwaitable(1234)
1234

AFAICS you used to need __iter__ so that you could yield from, back in the days when async wasn't a keyword (a = yield from b instead of a = await b.) If I try to use my class in an old-style coroutine:

>>> import asyncio
>>> @asyncio.coroutine
... def f():
...     val = yield from DummyAwaitable(5678)
...     return val
... 
<console>:2: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
>>> await f()
Traceback (most recent call last):
  File "/usr/lib64/python3.8/concurrent/futures/_base.py", line 444, in result
    return self.__get_result()
  File "/usr/lib64/python3.8/concurrent/futures/_base.py", line 389, in __get_result
    raise self._exception
  File "<console>", line 1, in <module>
  File "<console>", line 3, in f
TypeError: 'DummyAwaitable' object is not iterable

But if we define __iter__:

>>> class OldDummy(DummyAwaitable):
...     def __iter__(self):
...         return self
... 
>>> @asyncio.coroutine
... def g():
...     val = yield from OldDummy(5678)
...     return val
... 
<console>:2: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
>>>     
>>> await g()
5678

[DMRobertson]

(Tbh we could probably use asyncio.Future instead of writing our own)

[erikjohnston]

(Tbh we could probably use asyncio.Future instead of writing our own)

Alas, Future docs say:

Deprecated since version 3.10: Deprecation warning is emitted if loop is not specified and there is no running event loop.

[DMRobertson]

I think this agrees with Rich prior to d15d241?

[DMRobertson]

I couldn't resist trying to simplify this. Needs careful checking though, I find all this confusing.

C.f. a0aef0b

[DMRobertson]

Err, being honest it's because I couldn't work out how to annotate the __iter__ and __next__ in a way that mypy was happy with. Can dig up the errors if you'd like.

I think this is equivalent though:

• __await__ is supposed to return an iterator
• an iterator is a thing with a __next__
• (so was __iter__ redundant? Might have needed it before the await syntax added in Py 3.5?)

[clokep]

I believe this is correct.