def get_handler(self, grant_type):
        """
        Return a function or method that is capable handling the ``grant_type``
        requested by the client or return ``None`` to indicate that this type
        of grant type is not supported, resulting in an error response.
        """
        if grant_type == 'authorization_code':
            return self.authorization_code
        elif grant_type == 'refresh_token':
            return self.refresh_token
        elif grant_type == 'password':
            return self.password
        return None
Return a function or method that is capable handling the ``grant_type``
        requested by the client or return ``None`` to indicate that this type
        of grant type is not supported, resulting in an error response.