code: def get_fields(self, project=None, expand=None):
          
          route_values = {}
          if project is not None:
              route_values['project'] = self._serialize.url('project', project, 'str')
          query_parameters = {}
          if expand is not None:
              query_parameters['$expand'] = self._serialize.query('expand', expand, 'str')
          response = self._send(http_method='GET',
                                location_id='b51fd764-e5c2-4b9b-aaf7-3395cf4bdd94',
                                version='5.0',
                                route_values=route_values,
                                query_parameters=query_parameters)
          return self._deserialize('[WorkItemField]', self._unwrap_collection(response))
docs: GetFields.
          Returns information for all fields.
          :param str project: Project ID or project name
          :param str expand: Use ExtensionFields to include extension fields, otherwise exclude them. Unless the feature flag for this parameter is enabled, extension fields are always included.
          :rtype: [WorkItemField]