Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import copy

def main():
    h, w = map(int, input().split())
    a = [[j == "#" for j in input()] for _ in range(h)]


    # スタート地点をqueueに追加
    que = deque([(i, j) for i in range(h) for j in range(w) if a[i][j]])

    count = 0
    while True:
        count += 1
        nque = deque()
        while que:
            y, x = que.popleft()
            if x > 0 and not a[y][x-1]:
                a[y][x-1] = True
                nque.append((y,x-1))
            if x < w-1 and not a[y][x+1]:
                a[y][x+1] = True
                nque.append((y,x+1))
            if y > 0 and not a[y-1][x]:
                a[y-1][x] = True
                nque.append((y-1,x))
            if y < h-1 and not a[y+1][x]:
                a[y+1][x] = True
                nque.append((y+1,x))

        if nque:
            que = copy.copy(nque)
        else:
            break

    print(count - 1)

if __name__ == '__main__':
    main()

Fixed submission:
from collections import deque
import copy

def main():
    h, w = map(int, input().split())
    a = [[j == "#" for j in input()] for _ in range(h)]


    # スタート地点をqueueに追加
    que = deque([(i, j) for i in range(h) for j in range(w) if a[i][j]])

    count = 0
    while True:
        count += 1
        nque = deque()
        while que:
            y, x = que.popleft()
            if x > 0 and not a[y][x-1]:
                a[y][x-1] = True
                nque.append((y,x-1))
            if x < w-1 and not a[y][x+1]:
                a[y][x+1] = True
                nque.append((y,x+1))
            if y > 0 and not a[y-1][x]:
                a[y-1][x] = True
                nque.append((y-1,x))
            if y < h-1 and not a[y+1][x]:
                a[y+1][x] = True
                nque.append((y+1,x))

        if nque:
            que = copy.copy(nque)
        else:
            break

    print(count - 1)

if __name__ == '__main__':
    main()

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

que=deque([(i,j) for i in range(h) for j in range(w) if a[i][j]])
count=0
while True:
  count+=1
  nque=deque()
  while que:
    y,x=que.popleft()
    if x>0 and not a[y][x-1]:
      a[y][x-1]=True
      nque.append((y,x-1))
    if x<w-1 and not a[y][x+1]:
      a[y][x+1]=True
      nque.append((y,x+1))
    if y>0 and not a[y-1][x]:
      a[y-1][x]=True
      nque.append((y-1,x))
    if y<h-1 and not a[y+1][x]:
      a[y+1][x]=True
      nque.append((y+1,x))
  if nque:
    que=copy.copy(nque)
  else:
    break
print(count-1)

Fixed submission:
from collections import deque
import copy

def main():
    h, w = map(int, input().split())
    a = [[j == "#" for j in input()] for _ in range(h)]


    # スタート地点をqueueに追加
    que = deque([(i, j) for i in range(h) for j in range(w) if a[i][j]])

    count = 0
    while True:
        count += 1
        nque = deque()
        while que:
            y, x = que.popleft()
            if x > 0 and not a[y][x-1]:
                a[y][x-1] = True
                nque.append((y,x-1))
            if x < w-1 and not a[y][x+1]:
                a[y][x+1] = True
                nque.append((y,x+1))
            if y > 0 and not a[y-1][x]:
                a[y-1][x] = True
                nque.append((y-1,x))
            if y < h-1 and not a[y+1][x]:
                a[y+1][x] = True
                nque.append((y+1,x))

        if nque:
            que = copy.copy(nque)
        else:
            break

    print(count - 1)

if __name__ == '__main__':
    main()