docs: def retrieve(pdb_id, cache_dir = None):
          '''Creates a PDBML object by using a cached copy of the files if they exists or by retrieving the files from the RCSB.'''
  
          pdb_contents = None
          xml_contents = None
          pdb_id = pdb_id.upper()
  
          if cache_dir:
              # Check to see whether we have a cached copy of the PDB file
              filename = os.path.join(cache_dir, "%s.pdb" % pdb_id)
              if os.path.exists(filename):
                  pdb_contents = read_file(filename)
  
              # Check to see whether we have a cached copy of the XML file
              filename = os.path.join(cache_dir, "%s.xml" % pdb_id)
              if os.path.exists(filename):
                  xml_contents = read_file(filename)
  
          # Get any missing files from the RCSB and create cached copies if appropriate
          if not pdb_contents:
              pdb_contents = rcsb.retrieve_pdb(pdb_id)
              if cache_dir:
                  write_file(os.path.join(cache_dir, "%s.pdb" % pdb_id), pdb_contents)
  
          if not xml_contents:
              xml_contents = rcsb.retrieve_xml(pdb_id)
              if cache_dir:
                  write_file(os.path.join(cache_dir, "%s.xml" % pdb_id), xml_contents)
  
          # Return the object
          return PDBML_slow(xml_contents, pdb_contents)