code: def inasafe_sub_analysis_summary_field_value(
          exposure_key, field, feature, parent):
      
      _ = feature, parent  # NOQA
      project_context_scope = QgsExpressionContextUtils.projectScope(
          QgsProject.instance())
      project = QgsProject.instance()
  
      key = ('{provenance}__{exposure}').format(
          provenance=provenance_multi_exposure_analysis_summary_layers_id[
              'provenance_key'],
          exposure=exposure_key)
      if not project_context_scope.hasVariable(key):
          return None
  
      analysis_summary_layer = project.mapLayer(
          project_context_scope.variable(key))
      if not analysis_summary_layer:
          return None
  
      index = analysis_summary_layer.fields().lookupField(field)
      if index < 0:
          return None
  
      feature = next(analysis_summary_layer.getFeatures())
      return feature[index]
docs: Retrieve a value from field in the specified exposure analysis layer.