Buggy python submission (verdict: Runtime Error):
import queue


def up(y, x, d):
    return y-1, x, d


def down(y, x, d):
    return y+1, x, d


def left(y, x, d):
    return y, x-1, d


def right(y, x, d):
    return y, x+1, d


def put_all(q, y, x, d):
    q.put((up(y, x, d)))
    q.put((down(y, x, d)))
    q.put((left(y, x, d)))
    q.put((right(y, x, d)))
    return


def put_not_all(q, y, x, d, distances):
    if distances[y-1][x] != -1:
        q.put((y-1, x, d))
    if distances[y+1][x] != -1:
        q.put((y+1, x, d))
    if distances[y][x-1] != -1:
        q.put((y, x-1, d))
    if distances[y][x+1] != -1:
        q.put((y, x+1, d))


def solve(maze, init_y, init_x, distances, H, W):
    if maze[init_y][init_x] == "#":
        return 0

    distances[init_y][init_x] = 0

    q = queue.Queue()
    put_all(q, init_y, init_x, 1)

    farthest = [0, init_y, init_x]
    while not q.empty():
        y, x, d = q.get()
        # print(x, y)
        if x < 0 or x >= W or y < 0 or y >= H:
            # print("continue")
            continue
        if maze[y][x] == "#":
            distances[y][x] = 0
            continue
        distance = distances[y][x]
        if distance == -1:
            distances[y][x] = d
            put_all(q, y, x, d+1)
            if d > farthest[0]:
                farthest = [d, y, x]

    return farthest


def main():
    H, W = map(int, input().split())
    maze = [input() for _ in range(H)]

    flag = False
    y = 0
    x = 0
    for i in range(H):
        for j in range(W):
            initial_map = [[-1 for _ in range(W)] for _ in range(H)]
            distance, y, x = solve(maze, i, j, initial_map, H, W)
            if distance > 0:
                flag = True
                break
        if flag:
            break
    initial_map = [[-1 for _ in range(W)] for _ in range(H)]
    max_distance, y, x = solve(maze, x, y, initial_map, H, W)

    print(max_distance)


if __name__ == '__main__':
    main()


Fixed submission:
def d_maze_master(INF=float('inf')):
    from collections import deque
    H, W = [int(i) for i in input().split()]
    Maze = [list(input()) for _ in range(H)]

    def bfs(start_row, start_col):
        diff = ((-1, 0), (0, 1), (1, 0), (0, -1))

        def is_inside(row, col):
            return 0 <= row < H and 0 <= col < W

        queue = deque([(start_row, start_col)])
        dist = [[INF for _ in range(W)] for _ in range(H)]
        dist[start_row][start_col] = 0
        while queue:
            row, col = queue.pop()
            for dr, dc in diff:
                nr, nc = row + dr, col + dc
                if is_inside(nr, nc) and Maze[nr][nc] != '#' and dist[nr][nc] == INF:
                    queue.appendleft((nr, nc))
                    dist[nr][nc] = dist[row][col] + 1
        return dist

    ans = 0
    for start_row in range(H):
        for start_col in range(W):
            if Maze[start_row][start_col] == '#':
                continue
            dist = bfs(start_row, start_col)
            for row in range(H):
                for col in range(W):
                    if Maze[row][col] != '#':
                        ans = max(ans, dist[row][col])
    return ans

print(d_maze_master())

Buggy python submission (verdict: Runtime Error):
import queue


def up(y, x, d):
    return y-1, x, d


def down(y, x, d):
    return y+1, x, d


def left(y, x, d):
    return y, x-1, d


def right(y, x, d):
    return y, x+1, d


def put_all(q, y, x, d):
    q.put((up(y, x, d)))
    q.put((down(y, x, d)))
    q.put((left(y, x, d)))
    q.put((right(y, x, d)))
    return


def put_not_all(q, y, x, d, distances):
    if distances[y-1][x] != -1:
        q.put((y-1, x, d))
    if distances[y+1][x] != -1:
        q.put((y+1, x, d))
    if distances[y][x-1] != -1:
        q.put((y, x-1, d))
    if distances[y][x+1] != -1:
        q.put((y, x+1, d))


def solve(maze, init_y, init_x, distances, H, W):
    if maze[init_y][init_x] == "#":
        return 0

    distances[init_y][init_x] = 0

    q = queue.Queue()
    put_all(q, init_y, init_x, 1)

    farthest = [0, init_y, init_x]
    while not q.empty():
        y, x, d = q.get()
        # print(x, y)
        if x < 0 or x >= W or y < 0 or y >= H:
            # print("continue")
            continue
        if maze[y][x] == "#":
            distances[y][x] = 0
            continue
        distance = distances[y][x]
        if distance == -1:
            distances[y][x] = d
            put_all(q, y, x, d+1)
            if d > farthest[0]:
                farthest = [d, y, x]

    return farthest


def main():
    H, W = map(int, input().split())
    maze = [input() for _ in range(H)]

    flag = False
    y = 0
    x = 0
    for i in range(H):
        for j in range(W):
            initial_map = [[-1 for _ in range(W)] for _ in range(H)]
            distance, y, x = solve(maze, i, j, initial_map, H, W)
            if distance > 0:
                flag = True
                break
        if flag:
            break
    initial_map = [[-1 for _ in range(W)] for _ in range(H)]
    max_distance, y, x = solve(maze, y, x, initial_map, H, W)

    print(max_distance)


if __name__ == '__main__':
    main()


Fixed submission:
def d_maze_master(INF=float('inf')):
    from collections import deque
    H, W = [int(i) for i in input().split()]
    Maze = [list(input()) for _ in range(H)]

    def bfs(start_row, start_col):
        diff = ((-1, 0), (0, 1), (1, 0), (0, -1))

        def is_inside(row, col):
            return 0 <= row < H and 0 <= col < W

        queue = deque([(start_row, start_col)])
        dist = [[INF for _ in range(W)] for _ in range(H)]
        dist[start_row][start_col] = 0
        while queue:
            row, col = queue.pop()
            for dr, dc in diff:
                nr, nc = row + dr, col + dc
                if is_inside(nr, nc) and Maze[nr][nc] != '#' and dist[nr][nc] == INF:
                    queue.appendleft((nr, nc))
                    dist[nr][nc] = dist[row][col] + 1
        return dist

    ans = 0
    for start_row in range(H):
        for start_col in range(W):
            if Maze[start_row][start_col] == '#':
                continue
            dist = bfs(start_row, start_col)
            for row in range(H):
                for col in range(W):
                    if Maze[row][col] != '#':
                        ans = max(ans, dist[row][col])
    return ans

print(d_maze_master())

Buggy python submission (verdict: Time Limit Exceeded):
import queue


def up(y, x, d):
    return y-1, x, d


def down(y, x, d):
    return y+1, x, d


def left(y, x, d):
    return y, x-1, d


def right(y, x, d):
    return y, x+1, d


def put_all(q, y, x, d):
    q.put((up(y, x, d)))
    q.put((down(y, x, d)))
    q.put((left(y, x, d)))
    q.put((right(y, x, d)))
    return


def put_not_all(q, y, x, d, distances):
    if distances[y-1][x] != -1:
        q.put((y-1, x, d))
    if distances[y+1][x] != -1:
        q.put((y+1, x, d))
    if distances[y][x-1] != -1:
        q.put((y, x-1, d))
    if distances[y][x+1] != -1:
        q.put((y, x+1, d))


def solve(maze, init_y, init_x, distances, H, W):
    if maze[init_y][init_x] == "#":
        return 0

    distances[init_y][init_x] = 0

    q = queue.Queue()
    put_all(q, init_y, init_x, 1)

    farthest = 0
    while not q.empty():
        y, x, d = q.get()
        # print(x, y)
        if x < 0 or x >= W or y < 0 or y >= H:
            # print("continue")
            continue
        if maze[y][x] == "#":
            distances[y][x] = 0
            continue
        distance = distances[y][x]
        if distance == -1:
            distances[y][x] = d
            put_all(q, y, x, d+1)
            farthest = max(farthest, d)

    return farthest


def main():
    H, W = map(int, input().split())
    maze = [input() for _ in range(H)]

    max_distance = 0
    for i in range(H):
        for j in range(W):
            initial_map = [[-1 for _ in range(W)] for _ in range(H)]
            distance = solve(maze, i, j, initial_map, H, W)
            max_distance = max(distance, max_distance)
    print(max_distance)


if __name__ == '__main__':
    main()


Fixed submission:
def d_maze_master(INF=float('inf')):
    from collections import deque
    H, W = [int(i) for i in input().split()]
    Maze = [list(input()) for _ in range(H)]

    def bfs(start_row, start_col):
        diff = ((-1, 0), (0, 1), (1, 0), (0, -1))

        def is_inside(row, col):
            return 0 <= row < H and 0 <= col < W

        queue = deque([(start_row, start_col)])
        dist = [[INF for _ in range(W)] for _ in range(H)]
        dist[start_row][start_col] = 0
        while queue:
            row, col = queue.pop()
            for dr, dc in diff:
                nr, nc = row + dr, col + dc
                if is_inside(nr, nc) and Maze[nr][nc] != '#' and dist[nr][nc] == INF:
                    queue.appendleft((nr, nc))
                    dist[nr][nc] = dist[row][col] + 1
        return dist

    ans = 0
    for start_row in range(H):
        for start_col in range(W):
            if Maze[start_row][start_col] == '#':
                continue
            dist = bfs(start_row, start_col)
            for row in range(H):
                for col in range(W):
                    if Maze[row][col] != '#':
                        ans = max(ans, dist[row][col])
    return ans

print(d_maze_master())