Python 在相互依赖的类实例中使用 setter

Question

以下代码和运行时错误消息充分说明了问题。

class A():
def __init__(self, x=None):
    self._x = x

@property
def x(self):
    return self._x

@x.setter
def x(self, x):
    self._x = x


# Make two instances of class A
a = A()
b = A()
# Make each instance contain a reference to the other class instance by using
# a setter. Note that the value of the instance variable self._x is None at
# the time that the setter is called.
a.x(b)
b.x(a)

运行时结果:

Traceback (most recent call last):
  File "E:\Projects\Commands\Comands\test\commands\test.py", line 19, in <module>
    a.x(b)
TypeError: 'NoneType' object is not callable

我正在使用 Python 3.7.4 运行。

Answer 1

a.x(b) 将:

• 得到a.x——此时是None
• 调用 None(b) -- 这是错误的来源，因为 NoneType 不可调用

要使用 setter(它是一个描述符)，您需要进行属性分配:

a.x = b
b.x = a