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Using subprocesses involves the global event loop #421

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Using subprocesses involves the global event loop#421
@Tinche

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@Tinche
Tinche
opened on Sep 17, 2016

Hello,

I'm the maintainer of pytest-asyncio. Pytest-asyncio allows users to block access to the global event loop, and pass the event loop explicitly; this is considered by some in the community as a best-practices approach.

However I've just received a bug report this approach doesn't work with subprocess (on Unix at least). Consider this test:

@pytest.mark.asyncio(forbid_global_loop=True)
async def test_subprocess(event_loop):
    await asyncio.create_subprocess_shell('ls', loop=event_loop)


======================================================================== FAILURES ========================================================================
____________________________________________________________________ test_subprocess _____________________________________________________________________

event_loop = <_UnixSelectorEventLoop running=False closed=False debug=False>

    @pytest.mark.asyncio(forbid_global_loop=True)
    async def test_subprocess(event_loop):
>       await asyncio.create_subprocess_shell('ls', loop=event_loop)

tests/test_subprocess.py:7: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/usr/lib/python3.5/asyncio/subprocess.py:197: in create_subprocess_shell
    stderr=stderr, **kwds)
/usr/lib/python3.5/asyncio/base_events.py:1049: in subprocess_shell
    protocol, cmd, True, stdin, stdout, stderr, bufsize, **kwargs)
/usr/lib/python3.5/asyncio/unix_events.py:179: in _make_subprocess_transport
    with events.get_child_watcher() as watcher:
/usr/lib/python3.5/asyncio/events.py:647: in get_child_watcher
    return get_event_loop_policy().get_child_watcher()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <pytest_asyncio.plugin.ForbiddenEventLoopPolicy object at 0x7f2050255588>

    def get_child_watcher(self):
        "Get the watcher for child processes."
>       raise NotImplementedError
E       NotImplementedError

/usr/lib/python3.5/asyncio/events.py:538: NotImplementedError

Two comments:

  • can't the event_loop instance passed in be used for this functionality, instead of the global event loop policy? If an instance of the loop is provided, in my opinion asyncio shouldn't care about any globals.
  • event_loop_policy.get_child_watcher() is undocumented as far as I can see.

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