Tic-Tac-Toe

After reading this java.net article on making a nim playing computer program, I attempted to make a tic-tac-toe playing one in Python. This exercise also served as a refresher for my old AI mini-max lessons (although it would be difficult for me to forget the person who taught me).
I'm unable to reproduce the full code here due to space reasons.

class Node:
    def computeMiniMax(self):
        if len(self.children) == 0:
            winner = self.board.getWinPos()
            if winner == 'X': self.score =  1
            elif winner == 'O': self.score = -1
            else: self.score = 0
        else:
            for child in self.children:
                child.computeMiniMax()
            if self.player == 'X':
                self.score = max([node.score for node in self.children])
            else:
                self.score = min([node.score for node in self.children])

Here's where I build the game tree:

    def buildGameTree(self):

        winner = self.board.getWinPos()
        if winner == 'X' or winner == 'O' :
            return

        if self.player == 'X':
            newplayer = 'O'
        else:
            newplayer = 'X'

        for i in range(3):
            for j in range(3):
                char = self.board.getcell(i,j)
                if char == '.':
                    newboard = self.board.clone()
                    newboard.setcell(i, j, self.player)
                    node = Node(newboard, newplayer)
                    self.children.append(node)
                    node.buildGameTree()

Here is a sample run:

Who plays first? (1=computer 2=human)2
.|.|.
.|.|.
.|.|.
Your choice:1,0
Building game tree...
Done..
Computing node scores...
Done..
X|.|.
O|.|.
.|.|.
Your choice:1,1
X|.|.
O|O|X
.|.|.
Your choice:0,2
X|.|O
O|O|X
X|.|.
Your choice:2,2
X|X|O
O|O|X
X|.|O
Your choice:2,1
X|X|O
O|O|X
X|O|O
It is a draw !