code: def _get_envvar_value(self):
          
          envvar_name = None
  
          if self.envvar is True:
              envvar_name = self.envvar_name
              if envvar_name is None:
                  envvar_name = '_'.join(self.get_path()).upper()
          elif self.envvar:
              envvar_name = self.envvar
  
          if envvar_name and envvar_name in os.environ:
              return self.type.deserialize(os.environ[envvar_name])
          else:
              return not_set
docs: Internal helper to get item value from an environment variable
          if item is controlled by one, and if the variable is set.
  
          Returns not_set otherwise.