Buggy python submission (verdict: Time Limit Exceeded):
def bfs_longest(x,y):
    queuex = [x]
    queuey = [y]
    queuedis = [0]
    looked = [[x,y]]
    ans = [0]
    while(len(queuex)):
        nowx = queuex.pop(0)
        nowy = queuey.pop(0)
        nowdis = queuedis.pop(0)
        search = [[nowx+1,nowy],[nowx-1,nowy],[nowx,nowy+1],[nowx,nowy-1]]
        for i in range(len(search)):
            if not (search[i] in looked):
                if maze[search[i][0]][search[i][1]] == ".":
                    queuex.append(search[i][0])
                    queuey.append(search[i][1])
                    queuedis.append(nowdis+1)
                    ans.append(nowdis+1)
                    looked.append([search[i][0],search[i][1]])
    return max(ans)

h,w = map(int,input().split())
maze = [["#" for _ in range(w+2)]for p in range(h+2)]
anchor = []
for i in range(h):
    data = str(input())
    for j in range(w):
        maze[i+1][j+1] = data[j]
        if data[j] == ".":
            anchor.append([i+1,j+1])

answer = 0
for i in range(len(anchor)):
    answer = max(bfs_longest(anchor[i][0],anchor[i][1]),answer)
print(answer)

Fixed submission:
def bfs_longest(x,y):
    queuex = [x]
    queuey = [y]
    queuedis = [0]
    looked = [[x,y]]
    ans = [0]
    while(len(queuex)):
        nowx = queuex.pop(0)
        nowy = queuey.pop(0)
        nowdis = queuedis.pop(0)
        search = [[nowx+1,nowy],[nowx-1,nowy],[nowx,nowy+1],[nowx,nowy-1]]
        for i in range(len(search)):
            if not (search[i] in looked):
                if maze[search[i][0]][search[i][1]] == ".":
                    queuex.append(search[i][0])
                    queuey.append(search[i][1])
                    queuedis.append(nowdis+1)
                    ans.append(nowdis+1)
                    looked.append([search[i][0],search[i][1]])
    return ans[-1]

h,w = map(int,input().split())
maze = [["#" for _ in range(w+2)]for p in range(h+2)]
anchor = []
for i in range(h):
    data = str(input())
    for j in range(w):
        maze[i+1][j+1] = data[j]
        if data[j] == ".":
            anchor.append([i+1,j+1])

answer = 0
if (len(anchor)) == (h*w):
    print(h+w-2)
    exit()
for i in range(len(anchor)):
    answer = max(bfs_longest(anchor[i][0],anchor[i][1]),answer)
print(answer)

Buggy python submission (verdict: Time Limit Exceeded):
def bfs_longest(x,y):
    queuex = [x]
    queuey = [y]
    queuedis = [0]
    looked = [[x,y]]
    ans = [0]
    while(len(queuex)):
        nowx = queuex.pop(0)
        nowy = queuey.pop(0)
        nowdis = queuedis.pop(0)
        search = [[nowx+1,nowy],[nowx-1,nowy],[nowx,nowy+1],[nowx,nowy-1]]
        for i in range(len(search)):
            if not (search[i] in looked):
                if maze[search[i][0]][search[i][1]] == ".":
                    queuex.append(search[i][0])
                    queuey.append(search[i][1])
                    queuedis.append(nowdis+1)
                    ans.append(nowdis+1)
                    looked.append([search[i][0],search[i][1]])
    return max(ans)

h,w = map(int,input().split())
maze = [["#" for _ in range(w+2)]for p in range(h+2)]
anchor = []
for i in range(h):
    data = str(input())
    for j in range(w):
        maze[i+1][j+1] = data[j]
        if data[j] == ".":
            anchor.append([i+1,j+1])

answer = 0
for i in range(len(anchor)):
    answer = max(bfs_longest(anchor[i][0],anchor[i][1]),answer)
print(answer)

Fixed submission:
def bfs_longest(x,y):
    queuex = [x]
    queuey = [y]
    queuedis = [0]
    looked = [[x,y]]
    ans = [0]
    while(len(queuex)):
        nowx = queuex.pop(0)
        nowy = queuey.pop(0)
        nowdis = queuedis.pop(0)
        search = [[nowx+1,nowy],[nowx-1,nowy],[nowx,nowy+1],[nowx,nowy-1]]
        for i in range(len(search)):
            if not (search[i] in looked):
                if maze[search[i][0]][search[i][1]] == ".":
                    queuex.append(search[i][0])
                    queuey.append(search[i][1])
                    queuedis.append(nowdis+1)
                    ans.append(nowdis+1)
                    looked.append([search[i][0],search[i][1]])
    return ans[-1]

h,w = map(int,input().split())
maze = [["#" for _ in range(w+2)]for p in range(h+2)]
anchor = []
for i in range(h):
    data = str(input())
    for j in range(w):
        maze[i+1][j+1] = data[j]
        if data[j] == ".":
            anchor.append([i+1,j+1])

answer = 0
if (len(anchor)) == (h*w):
    print(h+w-2)
    exit()
for i in range(len(anchor)):
    answer = max(bfs_longest(anchor[i][0],anchor[i][1]),answer)
print(answer)