Buggy python submission (verdict: Runtime Error):

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from collections import deque
h,w = map(int,input().split(" "))
s = [input() for _ in range(h)]
 
ans = 0
direcion = [(1,0),(0,1),(-1,0),(0,-1)]
for i in range(h):
    for j in range(w):
        if s[i][j] == "#":
            continue
        que = deque([(j,i)])
        dist = [[0]*w for _ in range(h)]
        visited = set()
        visited.add((i,j))
        while que:
            j, i = que.popleft()
            for x,y in direcion:
                n_x,n_y = j+x,i+y
                if not 0 <= n_x < w: continue
                if not 0 <= n_y < h: continue
                if (n_y,n_x) in visited: continue
                if s[n_y][n_x] == "#": continue
                visited.add((n_y,n_x))
                dist[n_y][n_x] = dist[i][j] + 1
                que.append((n_x,n_y))
            ans = max(ans,max(max(row) for row in dist))
print(ans)

Fixed submission:
from collections import deque
h,w = map(int,input().split(" "))
s = [input() for _ in range(h)]
 
ans = 0
direcion = [(1,0),(0,1),(-1,0),(0,-1)]
for i in range(h):
    for j in range(w):
        if s[i][j] == "#":
            continue
        que = deque([(j,i)])
        dist = [[0]*w for _ in range(h)]
        visited = [[0]*w for _ in range(h)]
        visited[i][j] = 1
        while que:
            j, i = que.popleft()
            for x,y in direcion:
                n_x,n_y = j+x,i+y
                if not 0 <= n_x < w: continue
                if not 0 <= n_y < h: continue
                if visited[n_y][n_x]: continue
                if s[n_y][n_x] == "#": continue
                visited[n_y][n_x] = 1
                dist[n_y][n_x] = dist[i][j] + 1
                que.append((n_x,n_y))
        ans = max(ans,max(max(row) for row in dist))
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
h,w = map(int,input().split(" "))
s = [input() for _ in range(h)]

ans = 0
direcion = [(1,0),(0,1),(-1,0),(0,-1)]
for i in range(h):
    for j in range(w):
        if s[i][j] == "#":
            continue
        que = deque([(j,i)])
        dist = [[0]*w for _ in range(h)]
        visited = [[0]*w for _ in range(h)]
        visited[i][j] = 1
        while que:
            j, i = que.popleft()
            for x,y in direcion:
                n_x,n_y = j+x,i+y
                if not 0 <= n_x < w: continue
                if not 0 <= n_y < h: continue
                if visited[n_y][n_x]: continue
                if s[n_y][n_x] == "#": continue
                visited[n_y][n_x] = 1
                dist[n_y][n_x] = dist[i][j] + 1
                que.append((n_x,n_y))
            ans = max(ans,max(max(row) for row in dist))
print(ans)

Fixed submission:
from collections import deque
h,w = map(int,input().split(" "))
s = [input() for _ in range(h)]
 
ans = 0
direcion = [(1,0),(0,1),(-1,0),(0,-1)]
for i in range(h):
    for j in range(w):
        if s[i][j] == "#":
            continue
        que = deque([(j,i)])
        dist = [[0]*w for _ in range(h)]
        visited = [[0]*w for _ in range(h)]
        visited[i][j] = 1
        while que:
            j, i = que.popleft()
            for x,y in direcion:
                n_x,n_y = j+x,i+y
                if not 0 <= n_x < w: continue
                if not 0 <= n_y < h: continue
                if visited[n_y][n_x]: continue
                if s[n_y][n_x] == "#": continue
                visited[n_y][n_x] = 1
                dist[n_y][n_x] = dist[i][j] + 1
                que.append((n_x,n_y))
        ans = max(ans,max(max(row) for row in dist))
print(ans)