Python类继承，属性错误

Question

我用Python编写了一个StateMachine类，这样就可以从中继承。它的逻辑按预期工作，但我可以从名为StateMachineTest的派生StateMachine类中存在的StateMachineTest状态访问属性self.data。

生成以下错误：

Traceback (most recent call last):
File "/home/nbout/work/python/state_machine/main.py", line 68, in 
   main()
File "/home/nbout/work/python/state_machine/main.py", line 59, in main
   test = StateMachineTest()
File "/home/nbout/work/python/state_machine/main.py", line 47, in __init__
   StateMachine.__init__(self, Started())
File "/home/nbout/work/python/state_machine/state_machine.py", line 17, in _init__
   self.current_state.on_enter(self)
File "/home/nbout/work/python/state_machine/main.py", line 16, in on_enter
   print("Started: data:{}".format(sm_test.data))
Started: on_enter
AttributeError: 'StateMachineTest' object has no attribute 'data'
Started: on_exit

state_machine.py

class State:

    def on_enter(self, state_machine):
        pass

    def on_exit(self, state_machine):
        pass


class StateMachine:

    def __init__(self, start_state):
        self.current_state = start_state

        self.current_state.on_enter(self)

    def __del__(self):
        self.current_state.on_exit(self)

    def set_state(self, state):

        self.current_state.on_exit(self)
        self.current_state = state
        self.current_state.on_enter(self)

main.py

from state_machine import StateMachine
from state_machine import State


class StateTest(State):
    def pause(self, state_machine_test):
        pass

    def start(self, state_machine_test):
        pass


class Started(StateTest):
    def on_enter(self, sm_test):
        print("Started: on_enter")
        print("Started: data:{}".format(sm_test.data))

    def on_exit(self, sm_test):
        print("Started: on_exit")

    def pause(self, sm_test):
        print("Started: pause")
        sm_test.set_state(Paused())

    def start(self, sm_test):
        print("Started: start")


class Paused(StateTest):
    def on_enter(self, sm_test):
        print("Paused: on_enter")

    def on_exit(self, sm_test):
        print("Paused: on_exit")

    def pause(self, sm_test):
        print("Paused: pause")

    def start(self, sm_test):
        print("Paused: start")
        sm_test.set_state(Started())


class StateMachineTest(StateMachine):

    def __init__(self):
        StateMachine.__init__(self, Started())
        self.data = 10

    def pause(self):
        self.current_state.pause(self)

    def start(self):
        self.current_state.start(self)


def main():

    test = StateMachineTest()

    test.start()
    test.pause()
    test.pause()
    test.start()


if __name__ == '__main__':
    main()

Answer 1

哇，这段代码真是不必要的复杂。

问题似乎是您在StateMachineTest类的\uuuu init\uuuu中初始化了start，以便在设置self.data之前，其on\u enter运行。

您可以通过将self.data的赋值移动到super调用上方来解决此问题，但更好的办法是简化继承层次结构。

Answer 2

您的问题在于，StateMachine.\uuuu init\uuuuuuself（self，start（））要求已设置self的数据属性，但您只在下一行设置它（self.data=10）。切换这两条线路，然后重试！