Boggle in Python

This is a solver for the game “Boggle”. The aim of the game is to find as many words as possible in a 4-by-4 grid randomly filled with letters. You are allowed to go up, down, left, right, or diagonally, but not use the same letter more than once.

First, here’s a prefix tree, which is the ideal structure for looking up words one letter at a time:

class PrefixTree(object):
    def __init__(self, letter=None):
        self.letter = letter
        self.children = {}
        self.stop = False

    def add(self, word):
        if len(word):
            letter = word[0]
            word = word[1:]
            if letter not in self.children:
                self.children[letter] = PrefixTree(letter);
            return self.children[letter].add(word)
            self.stop = True
            return self

    def find_letter(self, letter):
        if letter not in self.children:
            return None
        return self.children[letter]

    def __repr__(self):
        return "PrefixTree(letter={0}, stop={1})".format(self.letter, self.stop)

Here’s the code for the game solver itself. It simply searches recursively starting at each cell in the game, and looks up the sequences of letters it builds up in the prefix tree:

from random import choice
from prefixtree import PrefixTree

class Boggle(object):
    def __init__(self, board=None):
        self.size = 4
        if board is None:
            self.board = []
            for i in range(0, self.size):
                for j in range(0, self.size):
            self.board = board

    def random_letter():
        return chr(choice(range(65,91)))

    def play(self, tree, found):
        for r in range(0, self.size):
            for c in range(0, self.size):
                self.search_r(tree, found, r, c)

    def search_r(self, tree, found, row, col, path=None, node=None, word=None):
        letter = self.board[row][col]
        if node is None or path is None or word is None:
            node = tree.find_letter(letter)
            path = [(row, col)]
            word = letter
            node = node.find_letter(letter)
            path.append((row, col))
            word = word + letter
        if node is None:
        elif node.stop:
        for r in range(row - 1, row + 2):
            for c in range(col - 1, col + 2):
                if (r >= 0 and r < self.size
                        and c >= 0 and c < self.size 
                        and not (r == row and c == col) 
                        and (r, c) not in path):
                    self.search_r(tree, found, r, c, path[:], node, word[:])

    def __repr__(self):
        return "Boggle(size={0}, board={1})".format(self.size, self.board)

I used this dictionary as a source of words. You may want to use a slightly smaller one.
Here is the main program:

def load_tree(tree):
    with open('dictionary.txt') as f:
        for line in f:
            word = line.rstrip().upper()

def main():
    boggle = Boggle()
    tree = PrefixTree()
    found = set(), found)
    for word in sorted(found):
        print word
    print boggle

if __name__ == '__main__':