Поиск пути, волновой алгоритм

def wave(r, c):
    q = [(r, c)]
    number = 1
    while q:
        new_q = []
        for r, c in q:
            m[r][c] = number
            for dr, dc in ((-1, 0), (1, 0), (0, -1), (0, 1)):
                rr = r + dr
                cc = c + dc
                if 0 <= rr < nr and 0 <= cc < nc:
                    if a[rr][cc] != 'X' and m[rr][cc] == 0:
                        new_q.append((rr, cc))
        number += 1
        q = new_q


def get_path(r, c):
    while m[r][c] > 1:
        next_res = None
        for dr, dc in ((-1, 0), (1, 0), (0, -1), (0, 1)):
            rr = r + dr
            cc = c + dc
            if 0 <= rr < nr and 0 <= cc < nc:
                if m[rr][cc] != 0:
                    if next_res is None or m[rr][cc] < next_res:
                        next_res = m[rr][cc]
                        next_r = rr
                        next_c = cc
        a[next_r][next_c] = '*'
        r, c = next_r, next_c


f = open('input.txt')
a = []
r = 0
for line in f.readlines():
    a.append(list(line.strip()))
    if 's' in line:
        start = (r, line.find('s'))
    if 'f' in line:
        finish = (r, line.find('f'))
    r += 1
f.close()

nc = len(a[0])
nr = len(a)
m = [[0 for c in range(nc)] for r in range(nr)]

wave(*start)
get_path(*finish)
r, c = finish
res = m[r][c] - 1
if res > 0:
    print(res)
else:
    print('Пути нет')
r, c = start
a[r][c] = 's'

for r in range(nr):
    for c in range(nc):
        print(a[r][c], end='')
    f.write()