code: def get_oa_version(doi):
      
      try:
          request = requests.get("%s%s" % (DISSEMIN_API, doi))
          request.raise_for_status()
          result = request.json()
          assert result["status"] == "ok"
          return result["paper"]["pdf_url"]
      except (AssertionError, ValueError, KeyError, RequestException):
          return None
docs: Get an OA version for a given DOI.
  
      .. note::
  
          Uses beta.dissem.in API.
  
      :param doi: A canonical DOI.
      :returns: The URL of the OA version of the given DOI, or ``None``.
  
      >>> get_oa_version('10.1209/0295-5075/111/40005')
      'http://arxiv.org/abs/1506.06690'