Python 类方法的装饰器问题

class Account:
    def __init__(self, **kwargs):
        [...]
        self._is_valid = True

    def valid_before_logout(self, func):
        def execute(*args, **kwargs):
            if self._is_valid:
                return func(*args, **kwargs)
            else:
                raise AccountOperatingError('the account is already invalidated or signed out')

        return execute

    @valid_before_logout
    def refresh(self):
        [...]

    @valid_before_logout
    def invalidate(self):
        self._is_valid = False
        [...]

    @valid_before_logout
    def signout(self):
        self._is_valid = False
        [...]

类Account里面有一个方法valid_before_logout用作装饰器，作用是在执行任何类方法之前检查self._is_valid，如果为False就抛出异常。

不过在导入这个模块的时候出错：

>>> import account
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "account.py", line 1, in <module>
    class Account:
  File "account.py", line 15, in <module>
    def refresh(self):
TypeError: valid_before_logout() missing 1 required positional argument: 'func'
>>>

请问这是什么原因？