code: def data_directory(self):
          
          return expand_path(self.get(property_name='data_directory',
                                      environment_variable='PIP_ACCEL_CACHE',
                                      configuration_option='data-directory',
                                      default='/var/cache/pip-accel' if is_root() else '~/.pip-accel'))
docs: The absolute pathname of the directory where pip-accel's data files are stored (a string).
  
          - Environment variable: ``$PIP_ACCEL_CACHE``
          - Configuration option: ``data-directory``
          - Default: ``/var/cache/pip-accel`` if running as ``root``, ``~/.pip-accel`` otherwise