docs: def _get_hash(self):
          """
          Create hash of the class.
  
          Hash should be unique for given ebook, so ISBN is main component of the
          hash if provided.
  
          Returns:
              str: Hash.
          """
          if self.optionals and self.optionals.ISBN:
              isbn = self.optionals.ISBN.replace("-", "")
  
              if len(isbn) <= 10:
                  return "97880" + isbn
  
              return isbn
  
          if self.optionals and self.optionals.EAN:
              return self.optionals.EAN
  
          return self.title + ",".join(map(lambda x: x.name, self.authors))