Safely set event loop to None

aiocontextvars/__init__.py

@@ -50,7 +50,7 @@ def create_task(loop, coro):
 
 
     def _patch_loop(loop):
-        if not hasattr(loop, '_orig_create_task'):
+        if loop and not hasattr(loop, '_orig_create_task'):
             loop._orig_create_task = loop.create_task
             loop.create_task = types.MethodType(create_task, loop)
         return loop

tests/test_aio.py

@@ -2,27 +2,29 @@
 import pytest
 from aiocontextvars import ContextVar
 
-pytestmark = pytest.mark.asyncio
-
 v = ContextVar('v')
 
 
+@pytest.mark.asyncio
 async def parallel(x):
     v.set(x)
     await asyncio.sleep(0.1)
     assert v.get() == x
 
 
+@pytest.mark.asyncio
 async def test_parallel():
     await asyncio.gather(*map(parallel, range(16)))
 
 
 # noinspection PyUnusedLocal
+@pytest.mark.asyncio
 async def test_parallel_with_inherit():
     await asyncio.gather(*map(parallel, range(16)))
 
 
 # noinspection PyUnusedLocal
+@pytest.mark.asyncio
 async def test_inherit():
     v.set('initial')
 
@@ -34,3 +36,8 @@ async def sub():
     await asyncio.sleep(0.1)
     await fut
     assert v.get('default') == 'update'
+
+
+def test_set_event_loop_none():
+    # Setting event loop to None should not raise an exception
+    asyncio.set_event_loop(None)

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35, py36, flake8
+envlist = py35, py36, py37, flake8
 
 [travis]
 python =