'Get mapper handler instance.
Returns:
cached handler instance as callable.'

def get_handler(self):

    if (self.__handler is None):
        resolved_spec = util.for_name(self.handler_spec)
        if isinstance(resolved_spec, type):
            self.__handler = resolved_spec()
        elif isinstance(resolved_spec, types.MethodType):
            self.__handler = getattr(resolved_spec.im_class(), resolved_spec.__name__)
        else:
            self.__handler = resolved_spec
    return self.__handler
