<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_friends_shake(self, before=None, after=None):
 """ Contrary to the endpoint naming, this resource is for a list of SharedFiles from your friends on mlkshk. Returns: List of SharedFiles. """

        if before and after:
            raise Exception("You cannot specify both before and after keys")

        endpoint = '/api/friends'

        if before:
            endpoint += '/before/{0}'.format(before)
        elif after:
            endpoint += '/after/{0}'.format(after)

        data = self._make_request("GET", endpoint=endpoint)
        return [SharedFile.NewFromJSON(sf) for sf in data['friend_shake']]