<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def method_selector_fn(self):
 """Gets the method selector from the config. """

        if callable(self.json_rpc_method):
            return self.json_rpc_method
        elif isinstance(self.json_rpc_method, (str,)):
            return lambda *_: self.json_rpc_method
        raise ValueError("``json_rpc_method`` config invalid.  May be a string or function")