Buggy python submission (verdict: Runtime Error):
from collections import deque
from itertools import chain

H, W = map(int, input().split())
M = [list(input()) for _ in range(H)]
inf = float('inf')


def flatten(list_of_lists):
    return chain.from_iterable(list_of_lists)


def bfs(start, end):
    distance = [[inf for i in range(W)] for j in range(H)]
    queue = deque((end, start))
    distance[end][start] = 0

    while queue:
        y, x = queue.pop()
        for i in range(0, 4):
            nx, ny = x + [1, 0, -1, 0][i], y + [0, 1, 0, -1][i]
            if 0 <= nx < W and 0 <= ny < H and M[ny][nx] != '#' and distance[ny][nx] == inf:
                queue.insert(0, (ny, nx))
                distance[ny][nx] = distance[y][x] + 1
    return distance


m = 0
for i in range(W):
    for j in range(H):
        if M[j][i] != "#":

            for length in list(flatten(bfs(i, j))):
                if length != inf and m < length:
                    m = length

print(m)


Fixed submission:

from itertools import chain

H, W = map(int, input().split())
M = [list(input()) for _ in range(H)]
inf = float('inf')

def flatten(list_of_lists):
    "Flatten one level of nesting"
    return chain.from_iterable(list_of_lists)


def bfs(start, end):
    distance = [[inf for i in range(W)] for j in range(H)]
    queue = [(end, start)]
    distance[end][start] = 0

    while queue:
        y, x = queue.pop()
        for i in range(0, 4):
            nx, ny = x + [1, 0, -1, 0][i], y + [0, 1, 0, -1][i]
            if 0 <= nx < W and 0 <= ny < H and M[ny][nx] != '#' and distance[ny][nx] == inf:
                queue.insert(0, (ny, nx))
                distance[ny][nx] = distance[y][x] + 1
    return distance


m = 0
for i in range(W):
    for j in range(H):
        if M[j][i] != "#":

            for length in list(flatten(bfs(i, j))):
                if length != inf and m < length:
                    m = length

print(m)

Buggy python submission (verdict: Runtime Error):
from math import inf
from itertools import chain

H, W = map(int, input().split())
M = [list(input()) for _ in range(H)]


def flatten(list_of_lists):
    "Flatten one level of nesting"
    return chain.from_iterable(list_of_lists)


def bfs(start, end):
    distance = [[inf for i in range(W)] for j in range(H)]
    queue = [(end, start)]
    distance[end][start] = 0

    while queue:
        y, x = queue.pop()
        for i in range(0, 4):
            nx, ny = x + [1, 0, -1, 0][i], y + [0, 1, 0, -1][i]
            if 0 <= nx < W and 0 <= ny < H and M[ny][nx] != '#' and distance[ny][nx] == inf:
                queue.insert(0, (ny, nx))
                distance[ny][nx] = distance[y][x] + 1
    return distance


m = 0
for i in range(W):
    for j in range(H):
        if M[j][i] != "#":

            for length in list(flatten(bfs(i, j))):
                if length != inf and m < length:
                    m = length

print(m)


Fixed submission:

from itertools import chain

H, W = map(int, input().split())
M = [list(input()) for _ in range(H)]
inf = float('inf')

def flatten(list_of_lists):
    "Flatten one level of nesting"
    return chain.from_iterable(list_of_lists)


def bfs(start, end):
    distance = [[inf for i in range(W)] for j in range(H)]
    queue = [(end, start)]
    distance[end][start] = 0

    while queue:
        y, x = queue.pop()
        for i in range(0, 4):
            nx, ny = x + [1, 0, -1, 0][i], y + [0, 1, 0, -1][i]
            if 0 <= nx < W and 0 <= ny < H and M[ny][nx] != '#' and distance[ny][nx] == inf:
                queue.insert(0, (ny, nx))
                distance[ny][nx] = distance[y][x] + 1
    return distance


m = 0
for i in range(W):
    for j in range(H):
        if M[j][i] != "#":

            for length in list(flatten(bfs(i, j))):
                if length != inf and m < length:
                    m = length

print(m)