[mutators] Prevent name collisions in mutators.

parlai/core/mutators.py

@@ -58,6 +58,11 @@ def register_mutator(name: str) -> Callable[[Type], Type]:
 
     def _inner(cls_: Type) -> Type:
         global MUTATOR_REGISTRY
+        if name in MUTATOR_REGISTRY and cls_ is not MUTATOR_REGISTRY[name]:
+            raise NameError(
+                "Mutators must be uniquely named, but detected two mutators with "
+                f"the name '{name}'."
+            )
         MUTATOR_REGISTRY[name] = cls_
         return cls_
 

tests/test_mutators.py

@@ -250,3 +250,26 @@ def test_not_sticky(self):
             second_epoch.append(teacher.act())
 
         assert all(f == s for f, s in zip(first_epoch, second_epoch))
+
+
+class TestUniqueness(unittest.TestCase):
+    """
+    Test that mutators cannot have duplicate names.
+    """
+
+    def test_uniqueness(self):
+        from parlai.core.mutators import register_mutator, Mutator
+
+        @register_mutator("test_unique_mutator")
+        class Mutator1(Mutator):
+            pass
+
+        # don't freak out if we accidentally register the exact same class twice
+        register_mutator("test_unique_mutator")(Mutator1)
+
+        # but do demand uniqueness
+        with self.assertRaises(NameError):
+
+            @register_mutator("test_unique_mutator")
+            class Mutator2(Mutator):
+                pass