1 files changed, 60 insertions, 22 deletions

diff --git a/Lib/asyncio/locks.py b/Lib/asyncio/locks.py
index 9fdb93745b..29c4434a62 100644
--- a/Lib/asyncio/locks.py
+++ b/Lib/asyncio/locks.py
@@ -9,6 +9,36 @@ from . import futures
 from . import tasks
 
 
+class _ContextManager:
+    """Context manager.
+
+    This enables the following idiom for acquiring and releasing a
+    lock around a block:
+
+        with (yield from lock):
+            <block>
+
+    while failing loudly when accidentally using:
+
+        with lock:
+            <block>
+    """
+
+    def __init__(self, lock):
+        self._lock = lock
+
+    def __enter__(self):
+        # We have no use for the "as ..."  clause in the with
+        # statement for locks.
+        return None
+
+    def __exit__(self, *args):
+        try:
+            self._lock.release()
+        finally:
+            self._lock = None  # Crudely prevent reuse.
+
+
 class Lock:
     """Primitive lock objects.
 
@@ -124,17 +154,29 @@ class Lock:
             raise RuntimeError('Lock is not acquired.')
 
     def __enter__(self):
-        if not self._locked:
-            raise RuntimeError(
-                '"yield from" should be used as context manager expression')
-        return True
+        raise RuntimeError(
+            '"yield from" should be used as context manager expression')
 
     def __exit__(self, *args):
-        self.release()
+        # This must exist because __enter__ exists, even though that
+        # always raises; that's how the with-statement works.
+        pass
 
     def __iter__(self):
+        # This is not a coroutine.  It is meant to enable the idiom:
+        #
+        #     with (yield from lock):
+        #         <block>
+        #
+        # as an alternative to:
+        #
+        #     yield from lock.acquire()
+        #     try:
+        #         <block>
+        #     finally:
+        #         lock.release()
         yield from self.acquire()
-        return self
+        return _ContextManager(self)
 
 
 class Event:
@@ -311,14 +353,16 @@ class Condition:
         self.notify(len(self._waiters))
 
     def __enter__(self):
-        return self._lock.__enter__()
+        raise RuntimeError(
+            '"yield from" should be used as context manager expression')
 
     def __exit__(self, *args):
-        return self._lock.__exit__(*args)
+        pass
 
     def __iter__(self):
+        # See comment in Lock.__iter__().
         yield from self.acquire()
-        return self
+        return _ContextManager(self)
 
 
 class Semaphore:
@@ -341,7 +385,6 @@ class Semaphore:
             raise ValueError("Semaphore initial value must be >= 0")
         self._value = value
         self._waiters = collections.deque()
-        self._locked = (value == 0)
         if loop is not None:
             self._loop = loop
         else:
@@ -349,7 +392,7 @@ class Semaphore:
 
     def __repr__(self):
         res = super().__repr__()
-        extra = 'locked' if self._locked else 'unlocked,value:{}'.format(
+        extra = 'locked' if self.locked() else 'unlocked,value:{}'.format(
             self._value)
         if self._waiters:
             extra = '{},waiters:{}'.format(extra, len(self._waiters))
@@ -357,7 +400,7 @@ class Semaphore:
 
     def locked(self):
         """Returns True if semaphore can not be acquired immediately."""
-        return self._locked
+        return self._value == 0
 
     @tasks.coroutine
     def acquire(self):
@@ -371,8 +414,6 @@ class Semaphore:
         """
         if not self._waiters and self._value > 0:
             self._value -= 1
-            if self._value == 0:
-                self._locked = True
             return True
 
         fut = futures.Future(loop=self._loop)
@@ -380,8 +421,6 @@ class Semaphore:
         try:
             yield from fut
             self._value -= 1
-            if self._value == 0:
-                self._locked = True
             return True
         finally:
             self._waiters.remove(fut)
@@ -392,23 +431,22 @@ class Semaphore:
         become larger than zero again, wake up that coroutine.
         """
         self._value += 1
-        self._locked = False
         for waiter in self._waiters:
             if not waiter.done():
                 waiter.set_result(True)
                 break
 
     def __enter__(self):
-        # TODO: This is questionable.  How do we know the user actually
-        # wrote "with (yield from sema)" instead of "with sema"?
-        return True
+        raise RuntimeError(
+            '"yield from" should be used as context manager expression')
 
     def __exit__(self, *args):
-        self.release()
+        pass
 
     def __iter__(self):
+        # See comment in Lock.__iter__().
         yield from self.acquire()
-        return self
+        return _ContextManager(self)
 
 
 class BoundedSemaphore(Semaphore):