Drop `async_generator` dependency

[Zac-HD]

The async_generator backport has served us well, but now that we've dropped Python 3.5 async generators exist, and having dropped Python 3.6 all the introspection functions we need are now in the standard library too.

I expect this to be a fairly small cleanup, but it's always nice to delete code and dependencies you don't need anymore 😄

[belm0]

async_generator includes aclosing(). While we've since added that to Python std library, it's only as of Python 3.10. So unless we're going to drop Python < 3.10 from trio, it's necessary to keep using async_generator, or make a local copy of aclosing().

https://async-generator.readthedocs.io/en/latest/reference.html#context-managers

[belm0]

oh, but I'm surprised to find that trio only uses aclosing() in some tests

perhaps it's enough to do something hacky, like disable relevant tests when running on Python < 3.10

trio/trio/_core/tests/test_asyncgen.py

Line 6 in 6754c74

from async_generator import aclosing

[Zac-HD]

We can condition the dependency and import on Python version, which will let people on 3.10 (👋 ) or later avoid the dependency... it's a little fiddly, but I do think that avoiding unneeded dependencies is pretty important at the bottom of the stack.

[belm0]

since it's only used in tests, just skip the relevant tests on python < 3.10

[jakkdl]

I think this will break ki protection (at least) on all old code that uses async_generator regardless of python version, since inspect.isasyncgenfunction does not recognise async generators created with that library.

@async_generator.async_generator
async def d():
    await async_generator.yield_()

async_generator.isasyncgenfunction(f) # true
inspect.isasyncgenfunction(f) # false

which is used in trio/_core/_ki.py line 144

I see a couple ways to handle this:

• keep the dependency for now, and print a warning (or raise an exception) if the output of the above functions differ
• drop the dependency, and inspect sys.modules for async_generator, in which case it gives warning/exception
• drop the dependency, but if sys.modules detects async_generator it imports it and checks if output of the above functions differ. If it does it gives warning (and continue working like previously) or raises exception.

[oremanj]

I think the best solution there is for Trio to conditionally import async_generator if it exists:

from inspect import isasyncgenfunction
try:
    from async_generator import isasyncgenfunction
except ImportError:
    pass

and not declare a dependency. If another package uses async_generator, then it will be installed and we can use it to check whether that other package has tried to KI protect a legacy async_generator function. If not, then we don't need it.

[Zac-HD]

We get away without conditional imports, so that we don't pull it in if async_generator is installed but not actually used:

from inspect import isasyncgenfunction

...
    # Use async_generator if it's been imported, otherwise inspect.
    is_agf = getattr(sys.modules.get("async_generator"), "isasyncgenfunction", isasyncgenfunction)
    if is_agf(fn):
        ...

[belm0]

I suggest just copying the isasyncgenfunction implementation, rather than bother to import async_generator. It's owned by python-trio, and the implementation will not be changing.

https://github.com/python-trio/async_generator/blob/9f0e08c762dc2b9e18a38e8dd714790a319ed836/async_generator/_impl.py#L451-L455