Properly preserve applicable_validators in extended validators.

Specifically, a validator extending an early JSON Schema draft where
siblings of `$ref` are ignored will properly ignore siblings in the
extended validator as well.

Closes: #1125

CHANGELOG.rst

@@ -1,3 +1,9 @@
+v4.18.3
+=======
+
+* Properly preserve ``applicable_validators`` in extended validators.
+  Specifically, validators extending early drafts where siblings of ``$ref`` were ignored will properly ignore siblings in the extended validator.
+
 v4.18.2
 =======
 

jsonschema/tests/test_validators.py

@@ -285,6 +285,23 @@ def id_of(schema):
         Derived = validators.extend(Original)
         self.assertEqual(Derived.ID_OF(Derived.META_SCHEMA), correct_id)
 
+    def test_extend_applicable_validators(self):
+        """
+        Extending a validator preserves its notion of applicable validators.
+        """
+
+        schema = {
+            "$defs": {"test": {"type": "number"}},
+            "$ref": "#/$defs/test",
+            "maximum": 1
+        }
+
+        draft4 = validators.Draft4Validator(schema)
+        self.assertTrue(draft4.is_valid(37))  # as $ref ignores siblings
+
+        Derived = validators.extend(validators.Draft4Validator)
+        self.assertTrue(Derived(schema).is_valid(37))
+
 
 class TestValidationErrorMessages(TestCase):
     def message_for(self, instance, schema, *args, **kwargs):

jsonschema/validators.py

@@ -219,6 +219,8 @@ class Validator:
         FORMAT_CHECKER = format_checker_arg
         ID_OF = staticmethod(id_of)
 
+        _APPLICABLE_VALIDATORS = applicable_validators
+
         schema: referencing.jsonschema.Schema = field(repr=reprlib.repr)
         _ref_resolver = field(default=None, repr=False, alias="resolver")
         format_checker: _format.FormatChecker | None = field(default=None)
@@ -570,6 +572,7 @@ def extend(
         type_checker=type_checker,
         format_checker=format_checker,
         id_of=validator.ID_OF,
+        applicable_validators=validator._APPLICABLE_VALIDATORS,
     )