"TypeError: argument of type 'NoneType' is not iterable"?
<p>Here is my code, which is a board game. I don't know why I'm getting this error:</p>

<blockquote>
  <p>TypeError: argument of type 'NoneType' is not iterable.</p>
</blockquote>

<p>The problem comes up in the <code>move_tiger()</code> function, but I use the same code in the <code>move_goat()</code> function and it works fine. Could someone please tell me what this error means in my case, because I didn't think I was trying to iterate over anything in the <code>move_tiger()</code> function. Thanks for your help.</p>

<pre><code>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
</code></pre>

<p>This code produces the following:</p>

<pre><code>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 &lt;module&gt;
    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
</code></pre>