Allow and correctly implement Semaphore(0).

gvanrossum · gvanrossum · commit f94c1135f12b · 2013-11-21T10:02:29.000-08:00
diff --git a/asyncio/locks.py b/asyncio/locks.py
@@ -348,12 +348,12 @@ class Semaphore:
 
     def __init__(self, value=1, bound=False, *, loop=None):
         if value < 0:
-            raise ValueError("Semaphore initial value must be > 0")
+            raise ValueError("Semaphore initial value must be >= 0")
         self._value = value
         self._bound = bound
         self._bound_value = value
         self._waiters = collections.deque()
-        self._locked = False
+        self._locked = (value == 0)
         if loop is not None:
             self._loop = loop
         else:
diff --git a/tests/test_locks.py b/tests/test_locks.py
@@ -684,6 +684,10 @@ def test_ctor_noloop(self):
         finally:
             events.set_event_loop(None)
 
+    def test_initial_value_zero(self):
+        sem = locks.Semaphore(0, loop=self.loop)
+        self.assertTrue(sem.locked())
+
     def test_repr(self):
         sem = locks.Semaphore(loop=self.loop)
         self.assertTrue(repr(sem).endswith('[unlocked,value:1]>'))
