Behavior Agent Implementation

class Behavior:

    def observe(self, x):
        """
        Receive an observation from the previous behavior. Return
        a or raise x to next behavior.
        """
        raise NotImplementedError

    def perform(self, a):
        """
        Get back an action from the next behavior. Return a or
        raise x to next behavior.
        """
        raise NotImplementedError


class Agent(Behavior):

    def __init__(self):
        self._behaviors = []

    def add(self, behavior):
        self._behaviors.add(behavior)

    def observe(self, x):
        return self._simulate(0, x, None)

    def perform(self, a):
        return self._simulate(len(self._behaviors) - 1, None, a)

    def _simulate(self, level, x, a):
        while True:
            if level < 0:
                return a
            if level >= len(self._behaviors):
                raise x
            level, x, a = self._step(level, x, a)

    def _step(self, level, x, a):
        assert (x is None) != (a is None)
        behavior = self._behaviors[level]
        try:
            if x is not None:
                a = behavior.observe(x)
            elif a is not None:
                a = behavior.perform(a)
            x = None
            level -= 1
        except Observation as x:
            a = None
            level += 1
        return level, x, a