我的Python狼羊菜脚本在Python 2.6中崩溃

Question

我正在尝试写一个脚本来解决“狼、白菜和山羊”的问题。每次我运行我的代码时，Python都会崩溃。我的代码有什么问题导致它无法正常工作呢？我想让代码运行后，能够打印出解决问题的每一步。

search.py

class Nodes:
    def succ(self, n):
        raise Exception, "Successor undefined"
    def start(self):
        raise Exception, "Start undefined"
    def goal(self, n):
        raise Exception, "Goal undefined"

wolf_cabbage_goat.py

import search # get the interface

def other_side(left_right):
    if left_right == 'left': return 'right'
    elif left_right == 'right': return 'left'

def safe(node):
    # the side without the farmer needs check,
    # the other is safe
    side_without_farmer = other_side(node['farmer'])

    lone_travelers = set(traveler for traveler in node if node[traveler] == side_without_farmer)

    # dangeroos animals
    unsafe = set(['wolf', 'goat']).issubset(lone_travelers) or\
             set(['goat', 'cabbage']).issubset(lone_travelers)
    return not unsafe

class Wolf_Cabbage_Goat(search.Nodes):
    def start(self):
        # farmer, wolf, cabbage, goat on one side
        return { 'farmer': 'left',
                 'wolf': 'left',
                 'goat': 'left',
                 'cabbage': 'left' }

    def goal(self, node):
        # true if all moved to right
        return set(node[i] for i in node) == set(['right'])

    def succ(self, node):
        for traveler in node.keys() + ['']:
            # for all possible travelers, including none

            if traveler == 'farmer':
                continue
            # ignore, we'll deal separately with him

            new_node = node.copy()
            # create a safety copy

            # if somebody wants to travel, and the farmer is on the
            # same side, do it

            if traveler and new_node[traveler] == new_node['farmer']:
                new_node[traveler] = other_side(new_node[traveler])

            # the farmer always travels
            new_node['farmer'] = other_side(new_node['farmer'])

            # after a trip, the new state is ok if no one gets eaten
            if safe(new_node):
                yield new_node

depth_first_search.py

def depth_first_search(problem, node):
    if problem.goal(node): return [node]
    # base case
    for n_succ in problem.succ(node):
        sol = depth_first_search(problem, n_succ)
        if sol:
            # first path is returned
            return [node] + sol

wcg_run.py

import wolf_cabbage_goat
from depth_first_search import *

wcg = wolf_cabbage_goat.Wolf_Cabbage_Goat()
print depth_first_search(wcg, wcg.start())

Answer 1

如果你遇到“未解决的引用”这个错误，我觉得可能是因为以下原因：

one_travelers = set(traveler for traveler in node if node[traveler] == side_without_farmer)

# dangeroos animals
unsafe = set(['wolf', 'goat']).issubset(lone_travelers)

你声明了一个叫 one_travelers 的变量，但却使用了 lone_travelers。

在提问的时候，附上错误信息是个好习惯。此外，如果你使用像 PyCharm 这样的好工具，可以避免很多这样的错误（我个人很喜欢 PyCharm）。