We CannotDetermineSpecification for $schema kyes that aren't strs

(E.g. if something has such a key but isn't a JSON Schema).

docs/changes.rst

@@ -2,6 +2,11 @@
 Changelog
 =========
 
+v0.32.1
+-------
+
+* Make ``Specification.detect`` raise a ``CannotDetermineSpecification`` error even if passed a mapping with a ``$schema`` key that doesn't match JSON Schema dialect semantics (e.g. a non-string).
+
 v0.32.0
 -------
 

referencing/_core.py

@@ -42,14 +42,12 @@ def _detect_or_error(contents: D) -> Specification[D]:
         raise exceptions.CannotDetermineSpecification(contents)
 
     jsonschema_dialect_id = contents.get("$schema")  # type: ignore[reportUnknownMemberType]
-    if jsonschema_dialect_id is None:
+    if not isinstance(jsonschema_dialect_id, str):
         raise exceptions.CannotDetermineSpecification(contents)
 
     from referencing.jsonschema import specification_with
 
-    return specification_with(
-        jsonschema_dialect_id,  # type: ignore[reportUnknownArgumentType]
-    )
+    return specification_with(jsonschema_dialect_id)
 
 
 def _detect_or_default(

referencing/tests/test_core.py

@@ -975,6 +975,10 @@ def test_detect_with_no_discernible_information(self):
         with pytest.raises(exceptions.CannotDetermineSpecification):
             Specification.detect({"foo": "bar"})
 
+    def test_detect_with_non_URI_schema(self):
+        with pytest.raises(exceptions.CannotDetermineSpecification):
+            Specification.detect({"$schema": 37})
+
     def test_detect_with_no_discernible_information_and_default(self):
         specification = Specification.OPAQUE.detect({"foo": "bar"})
         assert specification is Specification.OPAQUE