docs: def current_codes_from_pdb():
      """ Get list of all PDB codes currently listed in the PDB.
  
      Returns
      -------
      pdb_codes : list(str)
          List of PDB codes (in lower case).
      """
      url = 'http://www.rcsb.org/pdb/rest/getCurrent'
      r = requests.get(url)
      if r.status_code == 200:
          pdb_codes = [x.lower() for x in r.text.split('"') if len(x) == 4]
      else:
          print('Request for {0} failed with status code {1}'.format(url, r.status_code))
          return
      return pdb_codes