Buggy python submission (verdict: Runtime Error):
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
 
import numpy as np
from heapq import heappop, heappush
 
H,W = map(int,readline().split())
grid = np.full((H+2,W+2),b'#',dtype='S1')
grid[1:-1,1:-1] = np.frombuffer(read(),'S1').reshape(H,-1)[:,:W]
INF = 400
def shortest(grid, start, cost=0):
    dist = np.array([[INF for _ in range(W+2)] for _ in range(H+2)])
    dx = [-1, 0, 1, 0]
    dy = [0, 1, 0, -1]
    st = [(cost, start)]
    dist[start[0]][start[1]]=0
    while(st):
        c, [sx, sy] = heappop(st)
        for a, b in zip(dx, dy):
            x = sx + a
            y = sy + b
            if grid[x][y]==b'#':
              continue
            dxy = dist[sx][sy] + 1
            if dist[x][y] <= dxy:
                continue
            dist[x][y] = dxy
            heappush(st, (dist[x][y], [x, y]))
    return dist

m=0
for i in range(1,H+1):
  for j in range(1,W+1):
    if grid[i,j]==b'#':
      continue
    res=shortest(grid, [3,1], cost=0)[1:-1,1:-1]
    res[res==400]=0
    tm=np.max(res)
    if m < tm:
      m=tm
print(m)

Fixed submission:
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
 
import numpy as np
from heapq import heappop, heappush
 
H,W = map(int,readline().split())
grid = np.full((H+2,W+2),b'#',dtype='S1')
grid[1:-1,1:-1] = np.frombuffer(read(),'S1').reshape(H,-1)[:,:W]
INF = 400
def shortest(grid, start, cost=0):
    dist = np.array([[INF for _ in range(W+2)] for _ in range(H+2)])
    dx = [-1, 0, 1, 0]
    dy = [0, 1, 0, -1]
    st = [(cost, start)]
    dist[start[0]][start[1]]=0
    while(st):
        c, [sx, sy] = heappop(st)
        for a, b in zip(dx, dy):
            x = sx + a
            y = sy + b
            if grid[x][y]==b'#':
              continue
            dxy = dist[sx][sy] + 1
            if dist[x][y] <= dxy:
                continue
            dist[x][y] = dxy
            heappush(st, (dist[x][y], [x, y]))
    return dist

m=0
for i in range(1,H+1):
  for j in range(1,W+1):
    if grid[i,j]==b'#':
      continue
    res=shortest(grid, [i,j], cost=0)[1:-1,1:-1]
    res[res==400]=0
    tm=np.max(res)
    if m < tm:
      m=tm
print(m)

Buggy python submission (verdict: Runtime Error):
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
 
import numpy as np
from heapq import heappop, heappush
 
H,W = map(int,readline().split())
grid = np.full((H+2,W+2),b'#',dtype='S1')
grid[1:-1,1:-1] = np.frombuffer(read(),'S1').reshape(H,-1)[:,:W]
INF = 400
def shortest(grid, start, cost=0):
    dist = np.array([[INF for _ in range(W+2)] for _ in range(H+2)])
    dx = [-1, 0, 1, 0]
    dy = [0, 1, 0, -1]
    st = [(cost, start)]
    dist[start[0]][start[1]]=0
    while(st):
        c, [sx, sy] = heappop(st)
        for a, b in zip(dx, dy):
            x = sx + a
            y = sy + b
            if grid[x][y]==b'#':
              continue
            dxy = dist[sx][sy] + 1
            if dist[x][y] <= dxy:
                continue
            dist[x][y] = dxy
            heappush(st, (dist[x][y], [x, y]))
    return dist

m=0
for i in range(1,H+1):
  for j in range(1,W+1):
    print(i,j)
    if grid[i,j]==b'#':
      continue
    res=shortest(grid, [3,1], cost=0)[1:-1,1:-1]
    res[res==400]=0
    tm=np.max(res)
    if m < tm:
      m=tm
print(m)

Fixed submission:
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
 
import numpy as np
from heapq import heappop, heappush
 
H,W = map(int,readline().split())
grid = np.full((H+2,W+2),b'#',dtype='S1')
grid[1:-1,1:-1] = np.frombuffer(read(),'S1').reshape(H,-1)[:,:W]
INF = 400
def shortest(grid, start, cost=0):
    dist = np.array([[INF for _ in range(W+2)] for _ in range(H+2)])
    dx = [-1, 0, 1, 0]
    dy = [0, 1, 0, -1]
    st = [(cost, start)]
    dist[start[0]][start[1]]=0
    while(st):
        c, [sx, sy] = heappop(st)
        for a, b in zip(dx, dy):
            x = sx + a
            y = sy + b
            if grid[x][y]==b'#':
              continue
            dxy = dist[sx][sy] + 1
            if dist[x][y] <= dxy:
                continue
            dist[x][y] = dxy
            heappush(st, (dist[x][y], [x, y]))
    return dist

m=0
for i in range(1,H+1):
  for j in range(1,W+1):
    if grid[i,j]==b'#':
      continue
    res=shortest(grid, [i,j], cost=0)[1:-1,1:-1]
    res[res==400]=0
    tm=np.max(res)
    if m < tm:
      m=tm
print(m)