Python class inheritance, attributeError

Question

I wrote a StateMachine class in Python so it could be inherited from. The logic of it work as expected but I can access the Attribute self.data from a State which is present in the derived StateMachine class named StateMachineTest.

The following error is generated:

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()

acdr · Accepted Answer · 2017-06-02 09:31:46Z

0

Your problem lies in the fact that StateMachine.__init__(self, Started()) requires self's data atttribute to already be set, but you only set it the next line (self.data = 10). Switch these two lines around and try again!

answered Jun 2, 2017 at 9:31

acdr

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1 Comment

nbout Over a year ago

This is the solution, thanks. I will Accept your answer as soon as StackOverflow let me...

Daniel Roseman · Accepted Answer · 2017-06-02 09:30:49Z

0

Wow, this code is really unnecessarily complicated.

The problem appears to be that you initialise Started in the __init__ of your StateMachineTest class, so that its on_enter runs before you have set self.data.

You might be able fix this by moving the assignment of self.data above the super call, but a much better bet would be to simplify your inheritance hierarchy.

answered Jun 2, 2017 at 9:30

Daniel Roseman

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1 Comment

nbout Over a year ago

Thanks, that was my problem. I used inheritance that way to enforced reusability of the StateMachine class

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