code: def _validate_order_book(self, book):
          
          if book not in self.order_books:
              raise InvalidOrderBookError(
                  'Invalid order book \'{}\'. Choose from {}.'
                  .format(book, tuple(self.order_books))
              )
docs: Check if the given order book is valid.
  
          :param book: Order book name.
          :type book: str | unicode
          :raise InvalidOrderBookError: If an invalid order book is given.