Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys, math, fractions, itertools, collections, copy, heapq


def solve(h: int, w: int, a: "List[str]"):
    a = [[j == "#" for j in a[i]] for i in range(h)]
    que = collections.deque([(i, j) for i in range(h) for j in range(w) if a[i][j]])
 
    count = 0
    while True:
        count += 1
        nque = collections.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)
    return


# Generated by 1.1.4 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    H = int(next(tokens))  # type: int
    W = int(next(tokens))  # type: int
    A = [ next(tokens) for _ in range(W) ]  # type: "List[str]"
    solve(H, W, A)

if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
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: Runtime Error):
#!/usr/bin/env python3
import sys, math, fractions, itertools, collections, copy, heapq


def solve(H: int, W: int, A: "List[str]"):
    import sys
    import numpy as np
    from scipy.ndimage.morphology import distance_transform_cdt
    INPUT = lambda: sys.stdin.readline().rstrip()
    MAP = lambda: map(int, INPUT().split())
    sys.setrecursionlimit(10 ** 9)

    A = np.array([[(Ai[j] != "#") for j in range(W)] for Ai in A])
    print(distance_transform_cdt(A, metric="taxicab").max())

    return


# Generated by 1.1.4 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    H = int(next(tokens))  # type: int
    W = int(next(tokens))  # type: int
    A = [ next(tokens) for _ in range(W) ]  # type: "List[str]"
    solve(H, W, A)

if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
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()