Use custom string to represent args_validator in exceptions (#130)

doubles/allowance.py

@@ -352,5 +352,7 @@ def _expected_argument_string(self):
 
         if self.args is _any and self.kwargs is _any:
             return 'any args'
+        elif self._custom_matcher:
+            return "custom matcher: '{}'".format(self._custom_matcher.__name__)
         else:
             return build_argument_repr_string(self.args, self.kwargs)

test/expect_test.py

@@ -10,6 +10,25 @@
 
 
 class TestExpect(object):
+    def test_with_args_validator_not_called(self):
+        subject = InstanceDouble('doubles.testing.User')
+
+        def arg_matcher(*args):
+            return True
+        expect(subject).method_with_varargs.with_args_validator(arg_matcher)
+        with raises(MockExpectationError) as e:
+            verify()
+        teardown()
+
+        assert re.match(
+            r"Expected 'method_with_varargs' to be called on "
+            r"<InstanceDouble of <class '?doubles.testing.User'?"
+            r"(?: at 0x[0-9a-f]{9})?> object at .+> "
+            r"with custom matcher: 'arg_matcher', but was not."
+            r" \(.*doubles/test/expect_test.py:\d+\)",
+            str(e.value)
+        )
+
     def test_with_args_validator(self):
         subject = InstanceDouble('doubles.testing.User')