vstinner@apu$ PYTHONWARNINGS=error ./python -Werror -m test -v test_asyncio -m test_debug_mode_interop
== CPython 3.9.0a0 (heads/master-dirty:00f6493084, Jun 17 2019, 21:50:32) [GCC 9.1.1 20190503 (Red Hat 9.1.1-1)]
== Linux-5.1.6-300.fc30.x86_64-x86_64-with-glibc2.29 little-endian
== cwd: /home/vstinner/prog/python/master/build/test_python_23510
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
Run tests sequentially
0:00:00 load avg: 0.73 [1/1] test_asyncio
test_debug_mode_interop (test.test_asyncio.test_tasks.CompatibilityTests) ... FAIL

======================================================================
FAIL: test_debug_mode_interop (test.test_asyncio.test_tasks.CompatibilityTests)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/home/vstinner/prog/python/master/Lib/test/test_asyncio/test_tasks.py", line 3355, in test_debug_mode_interop
    assert_python_ok("-c", code, PYTHONASYNCIODEBUG="1")
  File "/home/vstinner/prog/python/master/Lib/test/support/script_helper.py", line 157, in assert_python_ok
    return _assert_python(True, *args, **env_vars)
  File "/home/vstinner/prog/python/master/Lib/test/support/script_helper.py", line 143, in _assert_python
    res.fail(cmd_line)
  File "/home/vstinner/prog/python/master/Lib/test/support/script_helper.py", line 70, in fail
    raise AssertionError("Process return code is %d\n"
AssertionError: Process return code is 1
command line: ['/home/vstinner/prog/python/master/python', '-X', 'faulthandler', '-c', '\nimport asyncio\n\nasync def native_coro():\n    pass\n\n@asyncio.coroutine\ndef old_style_coro():\n    yield from native_coro()\n\nasyncio.run(old_style_coro())\n']

stdout:
---

---

stderr:
---

Traceback (most recent call last):
  File "<string>", line 8, in <module>
  File "/home/vstinner/prog/python/master/Lib/asyncio/coroutines.py", line 111, in coroutine
    warnings.warn('"@coroutine" decorator is deprecated since Python 3.8, use "async def" instead',
DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
---

Ran 1 test in 0.210s

FAILED (failures=1)
test test_asyncio failed
test_asyncio failed

== Tests result: FAILURE ==

1 test failed:
test_asyncio

Total duration: 598 ms
Tests result: FAILURE

It affects 3.8 as well since #13346 was merged and I guess the fix would be to wrap it with assertWarns(DeprecationWarning) context manager like other tests. Interesting, does PYTHONWARNINGS=error affects the interpreter spawn by assert_python_ok to change warnings level which -Werror doesn't do by itself?

Sorry, I was wrong PYTHONWARNINGS environment variable is still passed to subprocess created by assert_python_ok so using assertWarns doesn't suppress it. Perhaps the DeprecationWarning could be ignored in assert_python_ok like below with -W :

diff --git a/Lib/test/test_asyncio/test_tasks.py b/Lib/test/test_asyncio/test_tasks.py
index 22a49077d5..d5f44b8091 100644
--- a/Lib/test/test_asyncio/test_tasks.py
+++ b/Lib/test/test_asyncio/test_tasks.py
@@ -3352,7 +3352,9 @@ class CompatibilityTests(test_utils.TestCase):

             asyncio.run(old_style_coro())
         """)
-        assert_python_ok("-c", code, PYTHONASYNCIODEBUG="1")
+
+        assert_python_ok("-Wignore::DeprecationWarning", "-c", code,
+                         PYTHONASYNCIODEBUG="1")

 if __name__ == '__main__':

New changeset 186f709 by Miss Islington (bot) (Xtreak) in branch 'master':
bpo-37323: Suppress DeprecationWarning raised by @asyncio.coroutine (GH-14293)
186f709

New changeset 9b32b44 by Miss Islington (bot) in branch '3.8':
bpo-37323: Suppress DeprecationWarning raised by @asyncio.coroutine (GH-14293)
9b32b44