Rework how the never singleton is invoked

asyncio/core.py

@@ -94,11 +94,11 @@ def sleep(t):
 
 ################################################################################
 # "Never schedule" object"
-# Don't re-schedule the object that awaits the _never singleton.
+# Don't re-schedule the object that awaits _never().
 # For internal use only. Some constructs, like `await event.wait()`,
 # work by NOT re-scheduling the task which calls wait(), but by
 # having some other task schedule it later.
-class _Never:
+class _NeverSingletonGenerator:
     def __init__(self):
         self.state = None
         self.exc = StopIteration()
@@ -117,7 +117,10 @@ def __next__(self):
            self.exc.__traceback__ = None
            raise self.exc
 
-_never = _Never()
+def _never(sgen=_NeverSingletonGenerator()):
+    # assert sgen.state is None, "Check for a missing `await` in your code"
+    sgen.state = False
+    return sgen
 
 
 ################################################################################
@@ -150,13 +153,11 @@ def _dequeue(self, s):
 
     async def queue_read(self, s):
         self._enqueue(s, 0)
-        _never.state = False
-        await _never
+        await _never()
 
     async def queue_write(self, s):
         self._enqueue(s, 1)
-        _never.state = False
-        await _never
+        await _never()
 
     def remove(self, task):
         while True:

asyncio/event.py

@@ -62,8 +62,7 @@ async def wait(self):
             self.waiting.push_head(core.cur_task)
             # Set calling task's data to the event's queue so it can be removed if needed
             core.cur_task.data = self.waiting
-            core._never.state = False
-            await core._never
+            await core._never()
         return True
 
 

asyncio/lock.py

@@ -69,8 +69,7 @@ async def acquire(self):
             # Set calling task's data to the lock's queue so it can be removed if needed
             core.cur_task.data = self.waiting
             try:
-                core._never.state = False
-                await core._never
+                await core._never()
             except core.CancelledError as er:
                 if self.state == core.cur_task:
                     # Cancelled while pending on resume, schedule next waiting Task