code: def _save_tracker_uri_to_file(self):
          
          if not self.tracker_file_name:
              return
          f = None
          try:
              f = open(self.tracker_file_name, 'w')
              f.write(self.tracker_uri)
          except IOError, e:
              raise ResumableUploadException(
                  'Couldn\'t write URI tracker file (%s): %s.\nThis can happen'
                  'if you\'re using an incorrectly configured upload tool\n'
                  '(e.g., gsutil configured to save tracker files to an '
                  'unwritable directory)' %
                  (self.tracker_file_name, e.strerror),
                  ResumableTransferDisposition.ABORT)
          finally:
              if f:
                  f.close()
docs: Saves URI to tracker file if one was passed to constructor.