“TypeError:'NoneType'类型的参数不可iterable”？

Question

这是我的代码，这是一个棋盘游戏。我不知道为什么我会犯这个错误：

TypeError: argument of type 'NoneType' is not iterable.

这个问题出现在move_tiger()函数中，但是我在move_goat()函数中使用了相同的代码，它工作得很好。有人能告诉我这个错误在我的例子中意味着什么吗，因为我认为我没有试图遍历move_tiger()函数中的任何内容。谢谢你的帮助。

class Board:

    def __init__(self): 
        self.board = dict()
        # a, b, c, d, and e are keys
        self.board['a'] = ['T']
        self.board['b'] = ['0','T','0','0','T','0']
        self.board['c'] = ['0','0','0','0','0','0']
        self.board['d'] = ['0','0','0','0','0','0']
        self.board['e'] = ['0','0','0','0']
        self.phase = 'add'
        self.numgoats = 0

    def print_board(self):
        for letter in 'abcde':
            for vertex in self.board[letter]:
                print vertex,
            print

    def content(self, position):
        row=list(position)[0]
        column=list(position)[1]
        return self.board[row][int(column)]

    def _set(self, position, value):
        self.board[position[0]][position[1]] = value

    def neighbors(self, position):
        if position == ('a',0):
            return [('b',1),('b',2),('b',3),('b',4)]
        # bunch of extraneous elif's that all return something skipped
        elif position == ('e',3):
            return [('e',2),('d',4)]

    def add_goat(self, position):
        if self.phase == 'add':
            if self.content(position) == '0':
                self._set(position, 'G')
                self.numgoats+=1
                if self.numgoats==3:
                    self.phase = 'move'

    def move_goat(self, old, new):
        if self.phase!='move':
            print "invalid move; try again"
        elif self.content(old) == 'G' and self.content(new) == '0' and new in self.neighbors(old):
            self.board[old[0]][old[1]] = '0'
            self.board[new[0]][new[1]] = 'G'
        else:
            print "invalid move; try again"

    def move_tiger(self, old, new):
        if self.content(old) == 'T' and self.content(new) == '0' and new in self.neighbors(old):
                self.board[old[0]][old[1]] = '0'
                self.board[new[0]][new[1]] = 'T'
        else:
            print "invalid move; try again"


myboard = Board()
myboard.print_board()
myboard.add_goat(('b',0))
myboard.print_board()
myboard.add_goat(('c',0))
myboard.print_board()
myboard.add_goat(('d',0))
myboard.print_board()
myboard.move_goat(('c',0),('c',1))
myboard.print_board()
myboard.move_goat(('c',1),('b',5))
myboard.print_board()
myboard.move_tiger(('b',4),('b',3))
myboard.print_board

此代码生成以下内容：

T
0 T 0 0 T 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0
T
G T 0 0 T 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0
T
G T 0 0 T 0
G 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0
T
G T 0 0 T 0
G 0 0 0 0 0
G 0 0 0 0 0
0 0 0 0
T
G T 0 0 T 0
0 G 0 0 0 0
G 0 0 0 0 0
0 0 0 0
invalid move; try again
T
G T 0 0 T 0
0 G 0 0 0 0
G 0 0 0 0 0
0 0 0 0
Traceback (most recent call last):
  File "game2.py", line 110, in <module>
    myboard.move_tiger(('b',4),('b',3))
  File "game2.py", line 91, in move_tiger
    if self.content(old) == 'T' and self.content(new) == '0' and new in self.neighbors(old):
TypeError: argument of type 'NoneType' is not iterable

Answer 1

当old值为("b", 4)时发生异常，因为self.neighbors(old)返回None。 我不确定是否完全理解代码的作用，但也许您应该检查一下在neighbors方法中是否遗漏了一个case。

Answer 2

在Python中，所有函数（和方法）都隐式地返回None，除非它们在执行期间到达显式的return语句。似乎您的neighbours方法接收到一个position，它不被任何if-elif分支所覆盖。

尝试在neighbours中添加else分支，以便可以看到意外的position：

def neighbors(self, position):
    if position == ('a',0):
        return [('b',1),('b',2),('b',3),('b',4)]
    elif position[0] == ('b',0):
        return [('b',1),('c',0),('c',1)]
    # ...
    else:
        raise ValueError(position)