code: def ReadApprovalRequest(self, requestor_username, approval_id):
      
      try:
        return self.approvals_by_username[requestor_username][approval_id]
      except KeyError:
        raise db.UnknownApprovalRequestError("Can't find approval with id: %s" %
                                             approval_id)
docs: Reads an approval request object with a given id.