Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin/env python3
import sys
from collections import deque
INF = float("inf")


def solve(H: int, W: int, A: "List[str]"):
    # 黒の位置をx, y座標として持つ。黒からの距離も一緒に持つ
    black = []
    B = [[-1]*W for _ in range(H)]
    for i in range(H):
        for j in range(W):
            if A[i][j] == "#":
                black.append((i, j, 0))
                B[i][j] = 0

    q = deque()
    q.extend(black)

    while len(q) > 0:
        cy, cx, dis = q.popleft()
        for y, x in zip([cy-1, cy, cy+1, cy], [cx, cx-1, cx, cx+1]):
            if 0 <= y < H and 0 <= x < W and B[y][x] < 0:
                B[y][x] = dis+1
                q.append((y, x, dis+1))

    m = -INF
    for i in range(H):
        a = max(B[i])
        if m < a:
            m = a
    print(m)

    return


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(H)]  # type: "List[str]"
    solve(H, W, A)


if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys
from collections import deque
INF = float("inf")


def solve(H: int, W: int, A: "List[str]"):
    # 黒の位置をx, y座標として持つ。黒からの距離も一緒に持つ
    black = []
    B = [[-1]*W for _ in range(H)]
    for i in range(H):
        for j in range(W):
            if A[i][j] == "#":
                black.append((i, j, 0))
                B[i][j] = 0

    q = deque()
    q.extend(black)

    while len(q) > 0:
        cy, cx, dis = q.popleft()
        for y, x in zip([cy-1, cy, cy+1, cy], [cx, cx-1, cx, cx+1]):
            if 0 <= y < H and 0 <= x < W and B[y][x] < 0:
                B[y][x] = dis+1
                q.append((y, x, dis+1))

    m = -INF
    for i in range(H):
        a = max(B[i])
        if m < a:
            m = a
    print(m)

    return


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(H)]  # type: "List[str]"
    solve(H, W, A)


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys
from collections import deque
INF = float("inf")


def argmost(a, comp, ini):
    m, n = ini, -1
    for i, v in enumerate(a):
        if comp(m, v):
            m, n = v, i
    return m, n


def solve(H: int, W: int, A: "List[str]"):
    # 黒の位置をx, y座標として持つ。黒からの距離も一緒に持つ
    black = []
    B = [[0]*W for _ in range(H)]
    for i in range(H):
        for j in range(W):
            if A[i][j] == "#":
                black.append((i, j, 0))
            B[i][j] = 0 if A[i][j] == "#" else -INF  # 黒は1

    black.sort(key=lambda x: x[0])

    q = deque()
    q.extend(black)

    while len(q) > 0:
        cy, cx, dis = q.popleft()
        for y, x in zip([cy-1, cy, cy+1, cy], [cx, cx-1, cx, cx+1]):
            if 0 <= y < H and 0 <= x < W and B[y][x] < 0:
                B[y][x] = dis+1
                q.append((y, x, dis+1))

    m = -INF
    for i in range(H):
        a = max(B[i])
        if m < a:
            m = a
    print(m)

    return


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
import sys
from collections import deque
INF = float("inf")


def solve(H: int, W: int, A: "List[str]"):
    # 黒の位置をx, y座標として持つ。黒からの距離も一緒に持つ
    black = []
    B = [[-1]*W for _ in range(H)]
    for i in range(H):
        for j in range(W):
            if A[i][j] == "#":
                black.append((i, j, 0))
                B[i][j] = 0

    q = deque()
    q.extend(black)

    while len(q) > 0:
        cy, cx, dis = q.popleft()
        for y, x in zip([cy-1, cy, cy+1, cy], [cx, cx-1, cx, cx+1]):
            if 0 <= y < H and 0 <= x < W and B[y][x] < 0:
                B[y][x] = dis+1
                q.append((y, x, dis+1))

    m = -INF
    for i in range(H):
        a = max(B[i])
        if m < a:
            m = a
    print(m)

    return


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(H)]  # type: "List[str]"
    solve(H, W, A)


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys
from collections import deque
INF = float("inf")


def solve(H: int, W: int, A: "List[str]"):
    # 黒の位置をx, y座標として持つ。
    black = []
    B = [[0]*W for _ in range(H)]
    for i in range(H):
        for j in range(W):
            if A[i][j] == "#":
                black.append((i, j))
            B[i][j] = 0 if A[i][j] == "#" else -INF  # 黒は1

    black2 = black.copy()
    black.sort(key=lambda x: x[0])
    black2.sort(key=lambda x: x[1])

    gene = 0
    q = deque()
    q.extend(black)
    q2 = deque()
    counter = len(black)
    while True:
        gene += 1
        while len(q) > 0:
            y, x = q.popleft()
            for yy, xx in zip([y-1, y, y+1, x], [x, x-1, x, x+1]):
                if 0 <= yy < H and 0 <= xx < W and B[yy][xx] < 0:
                    B[yy][xx] = gene
                    q2.append((yy, xx))
                    counter += 1
        q = q2
        q2 = deque()
        if counter == H*W:
            break
    print(gene)
    return


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
import sys
from collections import deque
INF = float("inf")


def solve(H: int, W: int, A: "List[str]"):
    # 黒の位置をx, y座標として持つ。黒からの距離も一緒に持つ
    black = []
    B = [[-1]*W for _ in range(H)]
    for i in range(H):
        for j in range(W):
            if A[i][j] == "#":
                black.append((i, j, 0))
                B[i][j] = 0

    q = deque()
    q.extend(black)

    while len(q) > 0:
        cy, cx, dis = q.popleft()
        for y, x in zip([cy-1, cy, cy+1, cy], [cx, cx-1, cx, cx+1]):
            if 0 <= y < H and 0 <= x < W and B[y][x] < 0:
                B[y][x] = dis+1
                q.append((y, x, dis+1))

    m = -INF
    for i in range(H):
        a = max(B[i])
        if m < a:
            m = a
    print(m)

    return


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(H)]  # type: "List[str]"
    solve(H, W, A)


if __name__ == '__main__':
    main()