In 3.5 and up the asyncio test are pretty chatty with warnings. E.g. a recent run gave me this in 3.5:

./python.exe -m test.test_asyncio
.........................................................................................................../Users/guido/src/cpython/Lib/asyncio/selector_events.py:574: ResourceWarning: unclosed transport <_SelectorSslTransport closing fd=27>
  warnings.warn("unclosed transport %r" % self, ResourceWarning)
Task was destroyed but it is pending!
task: <Task pending coro=<BaseSelectorEventLoop._accept_connection2() done, defined at /Users/guido/src/cpython/Lib/asyncio/selector_events.py:192> wait_for=<Future pending cb=[Task._wakeup()]>>
.........................................................................Task was destroyed but it is pending!
task: <Task pending coro=<BaseSelectorEventLoop._accept_connection2() running at /Users/guido/src/cpython/Lib/asyncio/selector_events.py:227> wait_for=<Future finished exception=RuntimeError('Event loop is closed',)>>
/Users/guido/src/cpython/Lib/asyncio/selector_events.py:574: ResourceWarning: unclosed transport <_SelectorSocketTransport closing fd=26>
  warnings.warn("unclosed transport %r" % self, ResourceWarning)
...............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................returning true from eof_received() has no effect when using ssl
...returning true from eof_received() has no effect when using ssl
...................................................................................................................................................................................................................................................................ss
----------------------------------------------------------------------
Ran 939 tests in 25.032s

OK (skipped=2)

Another source of noise is this warning about a task being slow to respond. I briefly investigated this a while back. I concluded that it only happens when other tests are run before it; it does not happen when test_asyncio is run alone. It is caused by the garbage collector, because I was able to stop it by invoking gc.collect() at a particular point in the test.

$ make -s -j2 && LC_TIME= ./python -bWall -m test -j0
[. . .]
[ 36/399] test_bytes
[ 37/399] test_bz2 -- running: test_buffer (32 sec), test_asyncore (42 sec), test_asyncio (44 sec)
[ 38/399] test_buffer (33 sec) -- running: test_asyncore (43 sec), test_asyncio (45 sec)
[ 39/399] test_asyncio (44 sec) -- running: test_asyncore (43 sec)
Executing <Task finished coro=<CoroutineTests.test_async_def_wrapped.<locals>.start() done, defined at /media/disk/home/proj/python/cpython/Lib/test/test_asyncio/test_pep492.py:146> result=None created at /media/disk/home/proj/python/cpython/Lib/asyncio/base_events.py:323> took 0.177 seconds
returning true from eof_received() has no effect when using ssl
returning true from eof_received() has no effect when using ssl

> Executing <Task finished coro=<CoroutineTests.test_async_def_wrapped.<locals>.start() done, defined at /media/disk/home/proj/python/cpython/Lib/test/test_asyncio/test_pep492.py:146> result=None created at /media/disk/home/proj/python/cpython/Lib/asyncio/base_events.py:323> took 0.177 seconds

This one is trivial to fix: set loop.slow_callback_duration to 1.0
(second) or more in TestCase.set_event_loop() of asyncio.test_utils.

Or call gc.collect() in that TestCase.setUp()?

New messages seen in Issue 25693 and Issue 25748:

/tmp/cpython/Lib/threading.py:864: ResourceWarning: unclosed <socket.socket fd=24, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=0, laddr=('127.0.0.1', 58231)>
  self._target(*self._args, **self._kwargs)
. . .
/tmp/cpython/Lib/asyncio/base_events.py:379: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=False>
  warnings.warn("unclosed event loop %r" % self, ResourceWarning)

I think I've fixed all of these:

https://github.com/python/asyncio/commit/cd4fdbb9ccd7c41a4e7c6b416bb2481ab0e21e1c
https://github.com/python/asyncio/commit/1365ac3a37836c6ec50138df8d64a87cdd0ac494
https://github.com/python/asyncio/commit/ae30b2b4244f1526d422500a1313c0c3dfd72dfe
https://github.com/python/asyncio/commit/5672730e26c555485447694e576c98d2a49341b9

I also updated asyncio/runtests.py to print out all warnings:

https://github.com/python/asyncio/commit/f25127c24aa8ca94960b5c012eb72afcef14403a

Closing this issue.