<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 async def get_input_sender(self):
 """ Returns `input_sender`, but will make an API call to find the input sender unless it's already cached. """

        if self.input_sender is None and self._sender_id:
            await self._refetch_sender()
        return self._input_sender