don't blow up with IndexError when there are no more items left

Author: radix

effect/test_testing.py

@@ -302,3 +302,8 @@ def test_success(self):
         self.assertEqual(
             sync_perform(d, eff),
             (('performfoo', 'foo'), ('performbar', 'bar')))
+
+    def test_ran_out(self):
+        """When there are no more items left, None is returned."""
+        d = SequenceDispatcher([])
+        self.assertEqual(d('foo'), None)

effect/testing.py

@@ -264,6 +264,8 @@ def __init__(self, sequence):
         self.sequence = sequence
 
     def __call__(self, intent):
+        if len(self.sequence) == 0:
+            return
         exp_intent, func = self.sequence[0]
         if intent == exp_intent:
             self.sequence = self.sequence[1:]