Add support for referencing schemas across different drafts.

In other words, a draft 7 schema may reference a draft 2019 schema
(or vice versa), and now correctly apply the 2019 rules within the
referenced schema.

Refs: json-schema-org/JSON-Schema-Test-Suite#587

Author: Julian

CHANGELOG.rst

@@ -1,3 +1,9 @@
+v4.10.0
+-------
+
+* Add support for referencing schemas with ``$ref`` across different versions
+  of the specification than the referrer's
+
 v4.9.1
 ------
 

jsonschema/protocols.py

@@ -168,4 +168,14 @@ def evolve(self, **kwargs) -> "Validator":
         >>> validator = Draft202012Validator({})
         >>> validator.evolve(schema={"type": "number"})
         Draft202012Validator(schema={'type': 'number'}, format_checker=None)
+
+        The returned object satisfies the validator protocol, but may not
+        be of the same concrete class! In particular this occurs
+        when a :validator:`$ref` occurs to a schema with a different
+        :validator:`$schema` than this one (i.e. for a different draft).
+
+        >>> validator.evolve(
+        ...     schema={"$schema": Draft7Validator.META_SCHEMA["$id"]}
+        ... )
+        Draft7Validator(schema=..., format_checker=None)
         """

jsonschema/tests/test_jsonschema_test_suite.py

@@ -264,6 +264,7 @@ def leap_second(test):
     DRAFT7.format_tests(),
     DRAFT7.optional_tests_of(name="bignum"),
     DRAFT7.optional_tests_of(name="content"),
+    DRAFT7.optional_tests_of(name="cross-draft"),
     DRAFT7.optional_tests_of(name="float-overflow"),
     DRAFT7.optional_tests_of(name="non-bmp-regex"),
     Validator=jsonschema.Draft7Validator,
@@ -323,6 +324,7 @@ def leap_second(test):
 TestDraft201909 = DRAFT201909.to_unittest_testcase(
     DRAFT201909.tests(),
     DRAFT201909.optional_tests_of(name="bignum"),
+    DRAFT201909.optional_tests_of(name="cross-draft"),
     DRAFT201909.optional_tests_of(name="float-overflow"),
     DRAFT201909.optional_tests_of(name="non-bmp-regex"),
     DRAFT201909.optional_tests_of(name="refOfUnknownKeyword"),
@@ -507,6 +509,7 @@ def leap_second(test):
 TestDraft202012 = DRAFT202012.to_unittest_testcase(
     DRAFT202012.tests(),
     DRAFT202012.optional_tests_of(name="bignum"),
+    DRAFT202012.optional_tests_of(name="cross-draft"),
     DRAFT202012.optional_tests_of(name="float-overflow"),
     DRAFT202012.optional_tests_of(name="non-bmp-regex"),
     DRAFT202012.optional_tests_of(name="refOfUnknownKeyword"),

jsonschema/validators.py

@@ -189,7 +189,6 @@ class Validator:
         schema = attr.ib(repr=reprlib.repr)
         resolver = attr.ib(default=None, repr=False)
         format_checker = attr.ib(default=None)
-        evolve = attr.evolve
 
         def __attrs_post_init__(self):
             if self.resolver is None:
@@ -203,6 +202,22 @@ def check_schema(cls, schema):
             for error in cls(cls.META_SCHEMA).iter_errors(schema):
                 raise exceptions.SchemaError.create_from(error)
 
+        def evolve(self, **changes):
+            schema = changes.setdefault("schema", self.schema)
+            NewValidator = validator_for(schema, default=Validator)
+
+            # Essentially reproduces attr.evolve, but may involve instantiating
+            # a different class than this one.
+            for field in attr.fields(Validator):
+                if not field.init:
+                    continue
+                attr_name = field.name  # To deal with private attributes.
+                init_name = attr_name if attr_name[0] != "_" else attr_name[1:]
+                if init_name not in changes:
+                    changes[init_name] = getattr(self, attr_name)
+
+            return NewValidator(**changes)
+
         def iter_errors(self, instance, _schema=None):
             if _schema is not None:
                 warnings.warn(