Add a deprecation warning to validate_for if schema doesn't exist

jsonschema/tests/test_validators.py

@@ -1100,7 +1100,7 @@ def test(self, format=format):
     setattr(TestBuiltinFormats, name, test)
 
 
-class TestValidatorFor(TestCase):
+class TestValidatorFor(SynchronousTestCase):
     def test_draft_3(self):
         schema = {"$schema": "http://json-schema.org/draft-03/schema"}
         self.assertIs(
@@ -1194,6 +1194,30 @@ def test_validator_for_jsonschema_default(self):
     def test_validator_for_custom_default(self):
         self.assertIs(validators.validator_for({}, default=None), None)
 
+    def test_warns_if_schema_specified_not_in_meta_schema(self):
+        self.assertWarns(
+            category=DeprecationWarning,
+            message=(
+                "This schema was not found but going to validate with latest draft. "
+                "This will raise an error in future. "
+            ),
+            # https://tm.tl/9363 :'(
+            filename=sys.modules[self.assertWarns.__module__].__file__,
+
+            f=validators.validator_for,
+            schema={u"$schema": 'unknownSchema'},
+            default={},
+        )
+
+    def test_doesnt_warns_if_schema_not_specified(self):
+        validators.validator_for(schema={}, default={}),
+        self.assertFalse(self.flushWarnings())
+
+    def test_latest_schema_used_if_schema_not_specified(self):
+        lastestSchema = validators.meta_schemas["http://json-schema.org/draft-07/schema#"]
+        schema = validators.validator_for(schema={}, default=lastestSchema)
+        self.assertEqual(schema, lastestSchema)
+
 
 class TestValidate(TestCase):
     def assertUses(self, schema, Validator):

jsonschema/validators.py

@@ -882,6 +882,15 @@ def validator_for(schema, default=_LATEST_VERSION):
             If unprovided, the default is to return
             the latest supported draft.
     """
-    if schema is True or schema is False:
+    if schema is True or schema is False or not u"$schema" in schema:
         return default
-    return meta_schemas.get(schema.get(u"$schema", u""), default)
+    if schema[u"$schema"] not in meta_schemas:
+        warn(
+            (
+                "This schema was not found but going to validate with latest draft. "
+                "This will raise an error in future. "
+            ),
+            DeprecationWarning,
+            stacklevel=2,
+        )
+    return meta_schemas.get(schema[u"$schema"], _LATEST_VERSION)