Do not allow a bare thenAnswer()

Because `when(m).foo().thenAnswer()` reads wrong when interpreted as
`return None` -- it reads like we *should* answer, (but what?), and
`None` is basically the `void` --, disallow its usage altogether for
clarity.

This is technically a breaking change.

CHANGES.txt

@@ -8,6 +8,7 @@ Release 1.5.5
 - Improved behavior of the ad-hoc methods of mocks.  (#92)
 - Make the shortcut `when(mock).foo().thenReturn()` officially work and just assume
   the user forgot the `None` as return value.
+- Disallow the shortcut `when(mock).foo().thenAnswer()` as it reads odd.
 
 
 

mockito/invocation.py

@@ -443,6 +443,8 @@ def thenRaise(self, *exceptions):
         return self
 
     def thenAnswer(self, *callables):
+        if not callables:
+            raise TypeError("No answer function provided")
         for callable in callables:
             answer = callable
             if self.discard_first_arg:

tests/when_interface_test.py

@@ -47,6 +47,12 @@ def testAssumeReturnNoneIfOmitted(self):
         assert dog.bark() is None
         assert dog.bark() == 42
 
+    def testRaiseIfAnswerIsOmitted(self):
+        dog = Dog()
+        with pytest.raises(TypeError) as exc:
+            when(dog).bark().thenAnswer()
+        assert str(exc.value) == "No answer function provided"
+
     def testAssumeRaiseExceptionIfOmitted(self):
         dog = Dog()
         when(dog).bark().thenRaise().thenReturn(42)