code: def bind_env(self, action, env):
          
          if env in self._env_actions:
              raise ValueError('Duplicate ENV variable: %s' % env)
          self._env_actions[env] = action
          action.env = env
docs: Bind an environment variable to an argument action.  The env
          value will traditionally be something uppercase like `MYAPP_FOO_ARG`.
  
          Note that the ENV value is assigned using `set_defaults()` and as such
          it will be overridden if the argument is set via `parse_args()`