Only use a default (remote retrieving registry) when none is provided.

In other words when someone does *not* do Validator(..., registry=...)

This both limits the case which we *do* still perform automatic remote
ref retrieval (in a way that still preserves backwards compatibility
given that the registry argument did not previously exist, so someone
using it must be doing something new) as well as fixes an issue with
creating a validator with a registry that has a retrieve function.

Previously the latter raised a ValueError (coming from referencing which
doesn't want you combining registries unless they agree about retrieval
functions. See the added test(s) for details).

Fixes the issue mentioned here:

#1065 (comment)

Refs: #1089

jsonschema/tests/test_validators.py

@@ -12,7 +12,9 @@
 import tempfile
 import warnings
 
+from referencing.jsonschema import DRAFT202012
 import attr
+import referencing.exceptions
 
 from jsonschema import (
     FormatChecker,
@@ -2146,6 +2148,32 @@ def validate():
         self.assertEqual((thread.is_alive(), failed), (False, []))
 
 
+class TestReferencing(TestCase):
+    def test_registry_with_retrieve(self):
+        def retrieve(uri):
+            return DRAFT202012.create_resource({"type": "integer"})
+
+        registry = referencing.Registry(retrieve=retrieve)
+        schema = {"$ref": "https://example.com/"}
+        validator = validators.Draft202012Validator(schema, registry=registry)
+
+        self.assertEqual(
+            (validator.is_valid(12), validator.is_valid("foo")),
+            (True, False),
+        )
+
+    def test_custom_registries_do_not_autoretrieve_remote_resources(self):
+        registry = referencing.Registry()
+        schema = {"$ref": "https://example.com/"}
+        validator = validators.Draft202012Validator(schema, registry=registry)
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            with self.assertRaises(referencing.exceptions.Unresolvable):
+                validator.validate(12)
+        self.assertFalse(w)
+
+
 class TestRefResolver(TestCase):
 
     base_uri = ""

jsonschema/validators.py

@@ -120,7 +120,7 @@ def _warn_for_remote_retrieve(uri: str):
         return referencing.Resource.from_contents(json.load(response))
 
 
-_DEFAULT_REGISTRY = SPECIFICATIONS.combine(
+_REMOTE_WARNING_REGISTRY = SPECIFICATIONS.combine(
     referencing.Registry(retrieve=_warn_for_remote_retrieve),  # type: ignore[call-arg]  # noqa: E501
 )
 
@@ -224,7 +224,7 @@ class Validator:
         format_checker: _format.FormatChecker | None = field(default=None)
         # TODO: include new meta-schemas added at runtime
         _registry: referencing.jsonschema.SchemaRegistry = field(
-            default=referencing.Registry(),
+            default=_REMOTE_WARNING_REGISTRY,
             kw_only=True,
             repr=False,
         )
@@ -269,7 +269,9 @@ def evolve(self, **changes):
 
         def __attrs_post_init__(self):
             if self._resolver is None:
-                registry = _DEFAULT_REGISTRY.combine(self._registry)
+                registry = self._registry
+                if registry is not _REMOTE_WARNING_REGISTRY:
+                    registry = SPECIFICATIONS.combine(registry)
                 resource = specification.create_resource(self.schema)
                 self._resolver = registry.resolver_with_root(resource)