Catch and expound upon the py38 proactor add_reader() not implemented exception

pytest_twisted.py

@@ -1,17 +1,30 @@
 import functools
 import inspect
 import sys
+import traceback
 import warnings
 
 import decorator
 import greenlet
 import pytest
+import six
 
 from twisted.internet import error, defer
 from twisted.internet.threads import blockingCallFromThread
 from twisted.python import failure
 
 
+class AddReaderNotImplementedError(Exception):
+    @classmethod
+    def build(cls):
+        return cls(
+            "Failed to install asyncio reactor.  The proactor was"
+            " used and is lacking the needed `.add_reader()` method"
+            " as of Python 3.8 on Windows."
+            "  https://twistedmatrix.com/trac/ticket/9766"
+        )
+
+
 class WrongReactorAlreadyInstalledError(Exception):
     pass
 
@@ -275,10 +288,27 @@ def init_qt5_reactor():
 def init_asyncio_reactor():
     from twisted.internet import asyncioreactor
 
-    _install_reactor(
-        reactor_installer=asyncioreactor.install,
-        reactor_type=asyncioreactor.AsyncioSelectorReactor,
-    )
+    try:
+        _install_reactor(
+            reactor_installer=asyncioreactor.install,
+            reactor_type=asyncioreactor.AsyncioSelectorReactor,
+        )
+    except NotImplementedError as e:
+        import asyncio
+
+        _, _, traceback_object = sys.exc_info()
+        stack_summary = traceback.extract_tb(traceback_object)
+        source_function_name = stack_summary[-1].name
+        event_loop = asyncio.get_event_loop()
+
+        if (
+                source_function_name == "add_reader"
+                and isinstance(event_loop, asyncio.ProactorEventLoop)
+        ):
+            six.raise_from(
+                value=AddReaderNotImplementedError.build(),
+                from_value=e,
+            )
 
 
 reactor_installers = {

setup.py

@@ -13,7 +13,7 @@
     author_email="[email protected]",
     url="https://github.com/pytest-dev/pytest-twisted",
     py_modules=["pytest_twisted"],
-    install_requires=["greenlet", "pytest>=2.3", "decorator"],
+    install_requires=["greenlet", "pytest>=2.3", "decorator", "six"],
     extras_require={"dev": ["pre-commit", "black"]},
     classifiers=[
         "Development Status :: 5 - Production/Stable",

testing/test_basic.py

@@ -49,6 +49,34 @@ def format_run_result_output_for_assert(run_result):
     )
 
 
+def proactor_add_reader_is_implemented():
+    try:
+        import asyncio
+
+        add_reader = asyncio.proactor_event_loop.add_reader
+
+        try:
+            add_reader(None, None, None)
+        except NotImplementedError:
+            return False
+        except:     # noqa: E722
+            return True
+    except:     # noqa: E722
+        return False
+
+
+def proactor_is_default():
+    try:
+        import asyncio
+
+        return (
+            asyncio.DefaultEventLoopPolicy
+            is asyncio.WindowsProactorEventLoopPolicy
+        )
+    except:     # noqa: E722
+        return False
+
+
 @pytest.fixture(name="default_conftest", autouse=True)
 def _default_conftest(testdir):
     testdir.makeconftest(textwrap.dedent("""
@@ -696,3 +724,38 @@ def test_succeed():
     testdir.makepyfile(test_file)
     rr = testdir.run(sys.executable, "-m", "pytest", "-v", *cmd_opts)
     assert "WrongReactorAlreadyInstalledError" in rr.stderr.str()
+
+
+@pytest.mark.skipif(
+    condition=sys.platform != 'win32',
+    reason="Relevant only on Windows",
+)
+@pytest.mark.skipif(
+    condition=proactor_add_reader_is_implemented(),
+    reason=(
+        "Relevant only if asyncio.ProactorEventLoop.add_reader()"
+        " is not implemented"
+    ),
+)
+@pytest.mark.skipif(
+    condition=not proactor_is_default(),
+    reason="Proactor is not the default event loop policy."
+)
+def test_add_reader_exception_expounded_upon(testdir, cmd_opts, request):
+    skip_if_reactor_not(request, "asyncio")
+
+    # block the default conftest.py
+    testdir.makeconftest("")
+    test_file = """
+    def test_succeed():
+        pass
+    """
+    testdir.makepyfile(test_file)
+    rr = testdir.run(sys.executable, "-m", "pytest", "-v", *cmd_opts)
+    assert all(
+        s in rr.stderr.str()
+        for s in [
+            "AddReaderNotImplementedError",
+            "https://twistedmatrix.com/trac/ticket/9766",
+        ]
+    )