Improve internal terminology for retrieval URI

The retrieval URI was internally called the 'ref base' or 'schema
URI', both of which suggest that it is definitively the base URI used
for reference resolution. This makes it confusing that adjusting this
URI might not change the base URI used.

Instead, call it properly 'retrieval URI'.

Base URI is newly introduced inside of the retrieve callback, after
receiving both the `$id` (if any) and the retrieval URI.

src/check_jsonschema/schema_loader/main.py

@@ -100,8 +100,8 @@ def _get_schema_reader(self) -> LocalSchemaReader | HttpSchemaReader:
                 f"detected parsed URL had an unrecognized scheme: {self.url_info}"
             )
 
-    def get_schema_ref_base(self) -> str | None:
-        return self.reader.get_ref_base()
+    def get_schema_retrieval_uri(self) -> str | None:
+        return self.reader.get_retrieval_uri()
 
     def get_schema(self) -> dict[str, t.Any]:
         return self.reader.read_schema()
@@ -113,7 +113,7 @@ def get_validator(
         format_opts: FormatOptions,
         fill_defaults: bool,
     ) -> jsonschema.Validator:
-        schema_uri = self.get_schema_ref_base()
+        retrieval_uri = self.get_schema_retrieval_uri()
         schema = self.get_schema()
 
         schema_dialect = schema.get("$schema")
@@ -123,7 +123,7 @@ def get_validator(
 
         # reference resolution
         # with support for YAML, TOML, and other formats from the parsers
-        reference_registry = make_reference_registry(self._parsers, schema_uri, schema)
+        reference_registry = make_reference_registry(self._parsers, retrieval_uri, schema)
 
         # get the correct validator class and check the schema under its metaschema
         validator_cls = jsonschema.validators.validator_for(schema)
@@ -147,7 +147,7 @@ def __init__(self, schema_name: str) -> None:
         self.schema_name = schema_name
         self._parsers = ParserSet()
 
-    def get_schema_ref_base(self) -> str | None:
+    def get_schema_retrieval_uri(self) -> str | None:
         return None
 
     def get_schema(self) -> dict[str, t.Any]:

src/check_jsonschema/schema_loader/readers.py

@@ -30,7 +30,7 @@ def __init__(self, filename: str) -> None:
         self.filename = str(self.path)
         self.parsers = ParserSet()
 
-    def get_ref_base(self) -> str:
+    def get_retrieval_uri(self) -> str:
         return self.path.as_uri()
 
     def _read_impl(self) -> t.Any:
@@ -55,7 +55,7 @@ def __init__(
             validation_callback=json.loads,
         )
 
-    def get_ref_base(self) -> str:
+    def get_retrieval_uri(self) -> str:
         return self.url
 
     def _read_impl(self) -> t.Any:

src/check_jsonschema/schema_loader/resolver.py

@@ -12,38 +12,46 @@
 
 
 def make_reference_registry(
-    parsers: ParserSet, schema_uri: str | None, schema: dict
+    parsers: ParserSet, retrieval_uri: str | None, schema: dict
 ) -> referencing.Registry:
+    id_attribute_: t.Any = schema.get("$id")
+    if isinstance(id_attribute_, str):
+        id_attribute: str | None = id_attribute_
+    else:
+        id_attribute = None
+
     schema_resource = referencing.Resource.from_contents(
         schema, default_specification=DRAFT202012
     )
     # mypy does not recognize that Registry is an `attrs` class and has `retrieve` as an
     # argument to its implicit initializer
     registry: referencing.Registry = referencing.Registry(  # type: ignore[call-arg]
-        retrieve=create_retrieve_callable(parsers, schema_uri)
+        retrieve=create_retrieve_callable(parsers, retrieval_uri, id_attribute)
     )
 
-    if schema_uri is not None:
-        registry = registry.with_resource(uri=schema_uri, resource=schema_resource)
-
-    id_attribute = schema.get("$id")
+    if retrieval_uri is not None:
+        registry = registry.with_resource(uri=retrieval_uri, resource=schema_resource)
     if id_attribute is not None:
         registry = registry.with_resource(uri=id_attribute, resource=schema_resource)
 
     return registry
 
 
 def create_retrieve_callable(
-    parser_set: ParserSet, schema_uri: str | None
+    parser_set: ParserSet, retrieval_uri: str | None, id_attribute: str | None
 ) -> t.Callable[[str], referencing.Resource[Schema]]:
+    base_uri = id_attribute
+    if base_uri is None:
+        base_uri = retrieval_uri
+
     def get_local_file(uri: str) -> t.Any:
         path = filename2path(uri)
         return parser_set.parse_file(path, "json")
 
     def retrieve_reference(uri: str) -> referencing.Resource[Schema]:
         scheme = urllib.parse.urlsplit(uri).scheme
-        if scheme == "" and schema_uri is not None:
-            full_uri = urllib.parse.urljoin(schema_uri, uri)
+        if scheme == "" and base_uri is not None:
+            full_uri = urllib.parse.urljoin(base_uri, uri)
         else:
             full_uri = uri