All the types in the types module are, being types, potentially callable to produce instances of that type. But they are in types rather than builtins precisely because it is not expected that they be called directly. They are bound to names in types primarily for isinstance checks, and possibly issubclass checks. So none of their signatures are documented in types. It would be an anomaly to add something special for MethodType. So my first impulse is to close this. Do you have a source for your statement?