Fix register_type not checking that the given type is a class

CHANGELOG.rst

@@ -32,6 +32,8 @@ Fixed
   <https://github.com/omni-us/jsonargparse/pull/818>`__).
 - ``default_config_files`` with settings for multiple subcommands not working
   correctly (`#819 <https://github.com/omni-us/jsonargparse/pull/819>`__).
+- ``register_type`` not checking that the given type is a class (`#820
+  <https://github.com/omni-us/jsonargparse/pull/820>`__).
 
 Changed
 ^^^^^^^

jsonargparse/typing.py

@@ -309,7 +309,7 @@ def deserializer(self, value):
 
 
 def register_type(
-    type_class: Any,
+    type_class: type,
     serializer: Callable = str,
     deserializer: Optional[Callable] = None,
     deserializer_exceptions: Union[type[Exception], tuple[type[Exception], ...]] = (
@@ -324,14 +324,16 @@ def register_type(
     """Registers a new type for use in jsonargparse parsers.
 
     Args:
-        type_class: The type object to be registered.
+        type_class: The class to be registered.
         serializer: Function that converts an instance of the class to a basic type.
         deserializer: Function that converts a basic type to an instance of the class. Default instantiates type_class.
         deserializer_exceptions: Exceptions that deserializer raises when it fails.
         type_check: Function to check if a value is of type_class. Gets as arguments the value and type_class.
         fail_already_registered: Whether to fail if type has already been registered.
         uniqueness_key: Key to determine uniqueness of type.
     """
+    if not inspect.isclass(type_class):
+        raise ValueError(f"Expected type_class to be a class, got {type_class!r}")
     type_handler = RegisteredType(type_class, serializer, deserializer, deserializer_exceptions, type_check)
     fail_already_registered = globals().get("_fail_already_registered", fail_already_registered)
     if not uniqueness_key and fail_already_registered and get_registered_type(type_class):

jsonargparse_tests/test_typing.py

@@ -376,6 +376,14 @@ def deserializer(v):
     pytest.raises(ValueError, lambda: register_type(datetime))  # different registration not okay
 
 
+def test_register_not_a_class_type_failure():
+    class SomeClass:
+        pass
+
+    with pytest.raises(ValueError, match="Expected type_class to be a class"):
+        register_type(Union[SomeClass, int])
+
+
 class RegisterOnFirstUse:
     pass