Ensure message_for allows only one error.

This prevents some nondeterminism (if there were multiple errors
that came out in different orders).

These tests don't really deal with multiple errors, so this
seems 'safe' from a convenience perspective to require.

Author: Julian

jsonschema/tests/test_validators.py

@@ -222,9 +222,15 @@ class TestValidationErrorMessages(TestCase):
     def message_for(self, instance, schema, *args, **kwargs):
         cls = kwargs.pop("cls", validators._LATEST_VERSION)
         cls.check_schema(schema)
-        with self.assertRaises(exceptions.ValidationError) as e:
-            cls(schema, *args, **kwargs).validate(instance)
-        return e.exception.message
+        validator = cls(schema, *args, **kwargs)
+        errors = list(validator.iter_errors(instance))
+        self.assertTrue(errors, msg=f"No errors were raised for {instance!r}")
+        self.assertEqual(
+            len(errors),
+            1,
+            msg=f"Expected exactly one error, found {errors!r}",
+        )
+        return errors[0].message
 
     def test_single_type_failure(self):
         message = self.message_for(instance=1, schema={"type": "string"})
@@ -484,7 +490,7 @@ def test_contains_too_few_both_constrained(self):
 
     def test_contains_too_many(self):
         message = self.message_for(
-            instance=["foo", "bar", "baz", "quux"],
+            instance=["foo", "bar", "baz"],
             schema={"contains": {"type": "string"}, "maxContains": 2},
         )
         self.assertEqual(
@@ -494,7 +500,7 @@ def test_contains_too_many(self):
 
     def test_contains_too_many_both_constrained(self):
         message = self.message_for(
-            instance=["foo"] * 7,
+            instance=["foo"] * 5,
             schema={
                 "contains": {"type": "string"},
                 "minContains": 2,