trio.open_file doesn't return context manager

[Nikratio]

The docs say:

Async file objects implement trio’s AsyncResource interface: you close them by calling aclose() instead of close (!!), and they can be used as async context managers.

but:

$ cat ~/test.py 
import trio

async def dostuff():
    async with trio.open_file('foo.txt', 'w') as fh:
          await foo.write('hello, world')

trio.run(dostuff)

$ python3 ~/test.py 
Traceback (most recent call last):
  File "/home/nikratio/test.py", line 7, in <module>
    trio.run(dostuff)
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/trio/_core/_run.py", line 1225, in run
    return result.unwrap()
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/trio/_core/_result.py", line 119, in unwrap
    raise self.error
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/trio/_core/_run.py", line 1334, in run_impl
    msg = task.coro.send(next_send)
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/trio/_core/_run.py", line 923, in init
    self.entry_queue.spawn()
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/trio/_util.py", line 109, in __aexit__
    await self._agen.asend(None)
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/async_generator/_impl.py", line 274, in asend
    return await self._do_it(self._it.send, value)
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/async_generator/_impl.py", line 290, in _do_it
    return await ANextIter(self._it, start_fn, *args)
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/async_generator/_impl.py", line 202, in send
    return self._invoke(self._it.send, value)
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/async_generator/_impl.py", line 209, in _invoke
    result = fn(*args)
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/trio/_core/_run.py", line 318, in open_nursery
    await nursery._nested_child_finished(nested_child_exc)
  File "/home/nikratio/.local/lib/python3.6/contextlib.py", line 99, in __exit__
    self.gen.throw(type, value, traceback)
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/trio/_core/_run.py", line 203, in open_cancel_scope
    yield scope
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/trio/_core/_multierror.py", line 144, in __exit__
    raise filtered_exc
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/trio/_core/_run.py", line 203, in open_cancel_scope
    yield scope
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/trio/_core/_run.py", line 318, in open_nursery
    await nursery._nested_child_finished(nested_child_exc)
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/trio/_core/_run.py", line 427, in _nested_child_finished
    raise MultiError(self._pending_excs)
  File "/home/nikratio/consulting/iofabric/venv/lib/python3.6/site-packages/trio/_core/_run.py", line 1334, in run_impl
    msg = task.coro.send(next_send)
  File "/home/nikratio/test.py", line 4, in dostuff
    async with trio.open_file('foo.txt', 'w') as fh:
AttributeError: __aexit__

[Nikratio]

$ python3 -c 'import trio; print(trio.version)'
0.3.0

[njsmith]

You want async with await open_file.

[njsmith]

Unfortunately it's very difficult to give a better error message here... #79 documents our attempts so far.

You ought to have at least gotten a "Warning: coroutine 'open_file' was never awaited" though. I don't know why you didn't! I don't get that warning here either. The lack of a warning seems like a CPython bug...

[njsmith]

Well, I filed bpo-32703; I think that's all we can do here for now.