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.

1 files changed, 11 insertions, 5 deletions

diff --git a/jsonschema/tests/test_validators.py b/jsonschema/tests/test_validators.py
index e3e4bbc..91d3819 100644
--- a/jsonschema/tests/test_validators.py
+++ b/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 @@ class TestValidationErrorMessages(TestCase):
 
     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 @@ class TestValidationErrorMessages(TestCase):
 
     def test_contains_too_many_both_constrained(self):
         message = self.message_for(
-            instance=["foo"] * 7,
+            instance=["foo"] * 5,
             schema={
                 "contains": {"type": "string"},
                 "minContains": 2,