Instrument use of custom --validator-class values

Implement the underlying arg-passing to set the `--validator-class`
from CLI arguments when creating a SchemaLoader (the class used
when `--schemafile` is used).

Author: sirosen

src/check_jsonschema/cli/main_command.py

@@ -290,6 +290,7 @@ def build_schema_loader(args: ParseResult) -> SchemaLoaderBase:
             args.cache_filename,
             args.disable_cache,
             base_uri=args.base_uri,
+            validator_class=args.validator_class,
         )
     else:
         raise NotImplementedError("no valid schema option provided")

src/check_jsonschema/schema_loader/main.py

@@ -56,18 +56,22 @@ def get_validator(
 
 
 class SchemaLoader(SchemaLoaderBase):
+    validator_class: type[jsonschema.protocols.Validator] | None = None
+
     def __init__(
         self,
         schemafile: str,
         cache_filename: str | None = None,
         disable_cache: bool = False,
         base_uri: str | None = None,
+        validator_class: type[jsonschema.protocols.Validator] | None = None,
     ) -> None:
         # record input parameters (these are not to be modified)
         self.schemafile = schemafile
         self.cache_filename = cache_filename
         self.disable_cache = disable_cache
         self.base_uri = base_uri
+        self.validator_class = validator_class
 
         # if the schema location is a URL, which may include a file:// URL, parse it
         self.url_info = None
@@ -132,9 +136,19 @@ def get_validator(
             self._parsers, retrieval_uri, schema
         )
 
-        # get the correct validator class and check the schema under its metaschema
-        validator_cls = jsonschema.validators.validator_for(schema)
-        validator_cls.check_schema(schema)
+        if self.validator_class is None:
+            # get the correct validator class and check the schema under its metaschema
+            validator_cls = jsonschema.validators.validator_for(schema)
+            validator_cls.check_schema(schema)
+        else:
+            # for a user-provided validator class, don't check_schema
+            # on the grounds that it might *not* be valid but the user wants to use
+            # their custom validator anyway
+            #
+            # in fact, there's no real guarantee that a user-provided
+            # validator_class properly conforms to the jsonschema.Validator protocol
+            # we *hope* that it does, but we can't be fully sure
+            validator_cls = self.validator_class
 
         # extend the validator class with default-filling behavior if appropriate
         if fill_defaults: