code: def _RegisterFlagByModuleId(self, module_id, flag):
      
      flags_by_module_id = self.FlagsByModuleIdDict()
      flags_by_module_id.setdefault(module_id, []).append(flag)
docs: Records the module that defines a specific flag.
  
      Args:
        module_id: An int, the ID of the Python module.
        flag: A Flag object, a flag that is key to the module.