word search in python

wordsearch.py

#!/usr/bin/env python3
# vim:sw=4:ts=4:et:
#
# based on https://www.tutorialspoint.com/word-search-in-python

BOARD = [
    ['A', 'B', 'C', 'E'],
    ['S', 'F', 'R', 'S'],
    ['A', 'D', 'E', 'E'],
    ['T', 'I', 'G', 'R'],
]

class Solution(object):
    def exist(self, board, word, res_board=None):
        n = len(board)
        m = len(board[0])
        for i in range(n):
            for j in range(m):
                if word[0] == board[i][j]:
                    if self.find(board, word, i, j, res_board=res_board):
                        return True
        return False

    def find(self, board, word, row, col, i=0, res_board=None):
        #print(f'find called: word={word} row={row} col={col} i={i}')
        if i == len(word):
            return True

        if row >= len(board) or row < 0 or col >= len(board[0]) or col < 0 or word[i] != board[row][col]:
            return False

        board[row][col] = '*'

        res = (
            self.find(board, word, row+1, col, i+1, res_board=res_board) or
            self.find(board, word, row-1, col, i+1, res_board=res_board) or
            self.find(board, word, row, col+1, i+1, res_board=res_board) or
            self.find(board, word, row, col-1, i+1, res_board=res_board)
        )

        board[row][col] = word[i]

        if res and res_board:
            res_board[row][col] = word[i]

        return res

    def print_board(self, board):
        for row in board:
            print(' '.join(row))

    def empty_board_from(self, board):
        res = []
        num_rows = len(board)
        num_cols = len(board[0])
        for i in range(num_rows):
            res.append([])
            for j in range(num_cols):
                res[i].append('.')
        return(res)

if __name__ == '__main__':
    ws = Solution()

    print('Source board:')
    ws.print_board(BOARD)

    for word in ('SEE', 'CAT', 'TIGR', 'TIGER', 'ERDT', 'AFER'):
        empty_board = ws.empty_board_from(BOARD)
        rc = ws.exist(BOARD, word, res_board=empty_board)
        print(f'Searching for {word}: {rc}')
        if rc:
            ws.print_board(empty_board)