[PYTHON] Implement a model with state and behavior (3) --Example of implementation by decorator

Last time (Implement a model with states and behaviors (2)), we implemented StopWatch with three states in two ways. .. This time, implement StopWatch using * decorator *.

First, describe the state transition with * decorator *. It becomes easy to understand whether it is a method that transitions to a state.

from enum import auto


def transit(state):
    def decorator(func):
        def inner(self, *args, **kwargs):
            self.start_stop, self.reset = self._TRANSIT[state]
            func(self, *args, **kwargs)
        return inner
    return decorator


class StopWatch:
    WAIT, PAUSE, MEASURE = auto(), auto(), auto()

    def __init__(self):
        self._TRANSIT = {StopWatch.WAIT: (self.start, lambda *args: None),
                         StopWatch.PAUSE: (self.start, self.reset_time),
                         StopWatch.MEASURE: (self.pause, lambda *args: None)}
        self._TRANSIT_REVERSED = {v: k for k, v in self._TRANSIT.items()}
        self.start_stop, self.reset = self._TRANSIT[StopWatch.WAIT]

    @property
    def state(self):
        return self._TRANSIT_REVERSED[self.start_stop, self.reset]

    @transit(MEASURE)
    def start(self):
        pass

    @transit(PAUSE)
    def pause(self):
        pass

    @transit(WAIT)
    def reset_time(self):
        pass

Furthermore, try cutting out the state transition table with a decorator. The behavior decorator represents the behavior in a certain state.

from enum import auto
from state_machine import behavior, transit


class StopWatch:
    WAIT, MEASURE, PAUSE = auto(), auto(), auto()

    def __init__(self):
        self.state = StopWatch.WAIT

    @behavior(WAIT, "start")
    @behavior(MEASURE, "pause")
    @behavior(PAUSE, "start")
    def start_stop(self):
        pass

    @behavior(PAUSE, "reset_time")
    def reset(self):
        pass

    @transit(MEASURE)
    def start(self):
        pass

    @transit(PAUSE)
    def pause(self):
        pass

    @transit(WAIT)
    def reset_time(self):
        pass

state_machine.py

from functools import wraps


def transit(state):
    def decorator(func):
        @wraps(func)
        def inner(self, *args, **kwargs):
            self.state = state
            func(self, *args, **kwargs)
        return inner
    return decorator


def behavior(state, function):
    def decorator(func):
        @wraps(func)
        def inner(self, *args, **kwargs):
            if self.state == state:
                getattr(self, function)(*args, **kwargs)
            else:
                func(self, *args, **kwargs)
        return inner
    return decorator

It is assumed that the state is kept in self.state, but since the same * decorator * can be used in other models, it was cut out as a module state_machine.

At the timing of processing the decorator, the function is still unbound, so the behavior is given as a string. You can also use the function __name__ to call a bound method at runtime. (Only applicable parts are shown below.) Note that if you have * decorator * that doesn't use functools.wrap (), it won't work as expected. (* decorator * doesn't rewrite the function __name__ when usingfunctools.wrap ().)

state_machine.py

def behavior(state, function):
...
            if self.state == state:
                getattr(self, function.__name__)(*args, **kwargs)
...

...
    @behavior(PAUSE, reset_time)
    def reset(self):
        pass
...