code: def get_bibtex(isbn_identifier):
      
      # Try to find the BibTeX using associated DOIs
      bibtex = doi.get_bibtex(to_doi(isbn_identifier))
      if bibtex is None:
          # In some cases, there are no DOIs for a given ISBN. In this case, try
          # to fetch bibtex directly from the ISBN, using a combination of
          # Google Books and worldcat.org results.
          bibtex = isbnlib.registry.bibformatters['bibtex'](
              isbnlib.meta(isbn_identifier, 'default'))
      return bibtex
docs: Get a BibTeX string for the given ISBN.
  
      :param isbn_identifier: ISBN to fetch BibTeX entry for.
      :returns: A BibTeX string or ``None`` if could not fetch it.
  
      >>> get_bibtex('9783161484100')
      '@book{9783161484100,\\n     title = {Berkeley, Oakland: Albany, Emeryville, Alameda, Kensington},\\n    author = {Peekaboo Maps},\\n      isbn = {9783161484100},\\n      year = {2009},\\n publisher = {Peek A Boo Maps}\\n}'