Support validator classes whose metaschema uses a different dialect.

In other words, one may author validator classes (via
jsonschema.validators.create or extend) whose meta schema defines schema
behavior using JSON Schema draft2020-12 but whose schemas are in its own
dialect.

Said again differently, the set of valid schemas for a validator class
may be governed by one draft, while the schema behavior itself is
governed by another.

Author: Julian

CHANGELOG.rst

@@ -1,3 +1,13 @@
+v4.13.0
+=======
+
+* Add support for creating validator classes whose metaschema uses a different
+  dialect than its schemas. In other words, they may use draft2020-12 to define
+  which schemas are valid, but the schemas themselves use draft7 (or a custom
+  dialect, etc.) to define which *instances* are valid. Doing this is likely
+  not something most users, even metaschema authors, may need, but occasionally
+  will be useful for advanced use cases.
+
 v4.12.1
 =======
 

jsonschema/tests/test_validators.py

@@ -197,6 +197,38 @@ def test_create_default_types(self):
             ),
         )
 
+    def test_check_schema_with_different_metaschema(self):
+        """
+        One can create a validator class whose metaschema uses a different
+        dialect than itself.
+        """
+
+        NoEmptySchemasValidator = validators.create(
+            meta_schema={
+                "$schema": validators.Draft202012Validator.META_SCHEMA["$id"],
+                "not": {"const": {}},
+            },
+        )
+        NoEmptySchemasValidator.check_schema({"foo": "bar"})
+
+        with self.assertRaises(exceptions.SchemaError):
+            NoEmptySchemasValidator.check_schema({})
+
+        NoEmptySchemasValidator({"foo": "bar"}).validate("foo")
+
+    def test_check_schema_with_different_metaschema_defaults_to_self(self):
+        """
+        A validator whose metaschema doesn't declare $schema defaults to its
+        own validation behavior, not the latest "normal" specification.
+        """
+
+        NoEmptySchemasValidator = validators.create(
+            meta_schema={"fail": [{"message": "Meta schema whoops!"}]},
+            validators={"fail": fail},
+        )
+        with self.assertRaises(exceptions.SchemaError):
+            NoEmptySchemasValidator.check_schema({})
+
     def test_extend(self):
         original = dict(self.Validator.VALIDATORS)
         new = object()

jsonschema/validators.py

@@ -215,7 +215,8 @@ def __attrs_post_init__(self):
 
         @classmethod
         def check_schema(cls, schema):
-            for error in cls(cls.META_SCHEMA).iter_errors(schema):
+            Validator = validator_for(cls.META_SCHEMA, default=cls)
+            for error in Validator(cls.META_SCHEMA).iter_errors(schema):
                 raise exceptions.SchemaError.create_from(error)
 
         def evolve(self, **changes):