Buggy python submission (verdict: Runtime Error):
field = []
w = 0
h = 0


def main():
    global field, w, h
    while True:
        h, w = map(int, input().split())
        if h == 0 and w == 0:
            break
        count = 0
        field = [[0] * w for _ in range(h)]
        for y in range(h):
            kinds = [_ for _ in input()]
            arrangement(kinds, y)

        for y in range(h):
            for x in range(w):
                if field[y][x] != -1:
                    dfs(field[y][x], x, y)
                    count += 1

        print(count)


def arrangement(kinds, y):
    global field
    for i, kind in enumerate(kinds):
        field[y][i] = kind


def dfs(kind, x, y):
    global field
    field[y][x] = -1
    for dx, dy in zip([0, 0, -1, 1], [1, -1, 0, 0]):
        next_x = x + dx
        next_y = y + dy
        if next_x < 0 or next_y < 0 or next_x >= w or next_y >= h:
            continue
        if field[next_y][next_x] != kind:
            continue
        dfs(field[next_y][next_x], next_x, next_y)


if __name__ == '__main__':
    main()

Fixed submission:
import sys
sys.setrecursionlimit(20000)

field = []
w = 0
h = 0


def main():
    global field, w, h
    while True:
        h, w = map(int, input().split())
        if h == 0 and w == 0:
            break
        count = 0
        field = []
        [field.append(list(input())) for _ in range(h)]
        for y in range(h):
            for x in range(w):
                if field[y][x] != -1:
                    dfs(field[y][x], x, y)
                    count += 1
        print(count)


def dfs(kind, x, y):
    global field
    field[y][x] = -1
    for dx, dy in zip([0, 0, -1, 1], [1, -1, 0, 0]):
        next_x = x + dx
        next_y = y + dy
        if next_x < 0 or next_y < 0 or next_x >= w or next_y >= h \
                or field[next_y][next_x] != kind:
            continue
        dfs(field[next_y][next_x], next_x, next_y)


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
field = []
w = 0
h = 0


def main():
    global field, w, h
    while True:
        h, w = map(int, input().split())
        if h == 0 and w == 0:
            break
        count = 0
        [field.append([_ for _ in input()]) for _ in range(h)]
        print(field)
        for y in range(h):
            for x in range(w):
                if field[y][x] != -1:
                    dfs(field[y][x], x, y)
                    count += 1
        print(count)


def arrangement(kinds, y):
    global field
    for i, kind in enumerate(kinds):
        field[y][i] = kind


def dfs(kind, x, y):
    global field
    print(x, y, field[y][x])
    field[y][x] = -1
    for dx, dy in zip([0, 0, -1, 1], [1, -1, 0, 0]):
        next_x = x + dx
        next_y = y + dy
        if next_x < 0 or next_y < 0 or next_x >= w or next_y >= h:
            continue
        if field[next_y][next_x] != kind:
            continue

        dfs(field[next_y][next_x], next_x, next_y)


if __name__ == '__main__':
    main()

Fixed submission:
import sys
sys.setrecursionlimit(20000)

field = []
w = 0
h = 0


def main():
    global field, w, h
    while True:
        h, w = map(int, input().split())
        if h == 0 and w == 0:
            break
        count = 0
        field = []
        [field.append(list(input())) for _ in range(h)]
        for y in range(h):
            for x in range(w):
                if field[y][x] != -1:
                    dfs(field[y][x], x, y)
                    count += 1
        print(count)


def dfs(kind, x, y):
    global field
    field[y][x] = -1
    for dx, dy in zip([0, 0, -1, 1], [1, -1, 0, 0]):
        next_x = x + dx
        next_y = y + dy
        if next_x < 0 or next_y < 0 or next_x >= w or next_y >= h \
                or field[next_y][next_x] != kind:
            continue
        dfs(field[next_y][next_x], next_x, next_y)


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
field = []
w = 0
h = 0


def main():
    global field, w, h
    while True:
        h, w = map(int, input().split())
        if h == 0 and w == 0:
            break
        count = 0
        [field.append([_ for _ in input()]) for _ in range(h)]
        for y in range(h):
            for x in range(w):
                if field[y][x] != -1:
                    dfs(field[y][x], x, y)
                    count += 1
        print(count)


def dfs(kind, x, y):
    global field
    field[y][x] = -1
    for dx, dy in zip([0, 0, -1, 1], [1, -1, 0, 0]):
        next_x = x + dx
        next_y = y + dy
        if next_x < 0 or next_y < 0 or next_x >= w or next_y >= h:
            continue
        if field[next_y][next_x] != kind:
            continue
        dfs(field[next_y][next_x], next_x, next_y)


if __name__ == '__main__':
    main()

Fixed submission:
import sys
sys.setrecursionlimit(20000)

field = []
w = 0
h = 0


def main():
    global field, w, h
    while True:
        h, w = map(int, input().split())
        if h == 0 and w == 0:
            break
        count = 0
        field = []
        [field.append(list(input())) for _ in range(h)]
        for y in range(h):
            for x in range(w):
                if field[y][x] != -1:
                    dfs(field[y][x], x, y)
                    count += 1
        print(count)


def dfs(kind, x, y):
    global field
    field[y][x] = -1
    for dx, dy in zip([0, 0, -1, 1], [1, -1, 0, 0]):
        next_x = x + dx
        next_y = y + dy
        if next_x < 0 or next_y < 0 or next_x >= w or next_y >= h \
                or field[next_y][next_x] != kind:
            continue
        dfs(field[next_y][next_x], next_x, next_y)


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
field = []
w = 0
h = 0


def main():
    global field, w, h
    while True:
        h, w = map(int, input().split())
        if h == 0 and w == 0:
            break
        count = 0
        field = []
        [field.append([_ for _ in input()]) for _ in range(h)]
        for y in range(h):
            for x in range(w):
                if field[y][x] != -1:
                    dfs(field[y][x], x, y)
                    count += 1
        print(count)


def dfs(kind, x, y):
    global field
    field[y][x] = -1
    for dx, dy in zip([0, 0, -1, 1], [1, -1, 0, 0]):
        next_x = x + dx
        next_y = y + dy
        if next_x < 0 or next_y < 0 or next_x >= w or next_y >= h:
            continue
        if field[next_y][next_x] != kind:
            continue
        dfs(field[next_y][next_x], next_x, next_y)


if __name__ == '__main__':
    main()

Fixed submission:
import sys
sys.setrecursionlimit(20000)

field = []
w = 0
h = 0


def main():
    global field, w, h
    while True:
        h, w = map(int, input().split())
        if h == 0 and w == 0:
            break
        count = 0
        field = []
        [field.append(list(input())) for _ in range(h)]
        for y in range(h):
            for x in range(w):
                if field[y][x] != -1:
                    dfs(field[y][x], x, y)
                    count += 1
        print(count)


def dfs(kind, x, y):
    global field
    field[y][x] = -1
    for dx, dy in zip([0, 0, -1, 1], [1, -1, 0, 0]):
        next_x = x + dx
        next_y = y + dy
        if next_x < 0 or next_y < 0 or next_x >= w or next_y >= h \
                or field[next_y][next_x] != kind:
            continue
        dfs(field[next_y][next_x], next_x, next_y)


if __name__ == '__main__':
    main()