Buggy python submission (verdict: Runtime Error):
While True:                                                                                      
    H, W = map(int, input().split())
    if H == 0 and W == 0:
        break
    countl = 0
 
    while True:
        if W == 0:
            break
        if countl == H:
            break
        elif countl % 2 == 0:
            countll = 0
            while True:
                if countll % 2 == 0:
                    print('#', end='')
                    countll += 1
                else:
                    print('.', end='')
                    countll += 1
                if countll == W:
                    print()
                    break
        elif countl % 2 == 1:
            countll = 0
            while True:
                if countll % 2 == 0:
                    print('.', end='')
                    countll += 1
                else:
                    print('#', end='')
                    countll += 1
                if countll == W:
                    print()
                    break
        countl += 1
    print()

Fixed submission:
while True:                                                                                      
    H, W = map(int, input().split())
    if H == 0 and W == 0:
        break
    countl = 0
 
    while True:
        if W == 0:
            break
        if countl == H:
            break
        elif countl % 2 == 0:
            countll = 0
            while True:
                if countll % 2 == 0:
                    print('#', end='')
                    countll += 1
                else:
                    print('.', end='')
                    countll += 1
                if countll == W:
                    print()
                    break
        elif countl % 2 == 1:
            countll = 0
            while True:
                if countll % 2 == 0:
                    print('.', end='')
                    countll += 1
                else:
                    print('#', end='')
                    countll += 1
                if countll == W:
                    print()
                    break
        countl += 1
    print()

Buggy python submission (verdict: Runtime Error):
While True:                                                                                      
    H, W = map(int, input().split())
    if H == 0 and W == 0:
        break
    countl = 0
 
    while True:
        if W == 0:
            break
        if countl == H:
            break
        elif countl % 2 == 0:
            countll = 0
            while True:
                if countll % 2 == 0:
                    print('#', end='')
                    countll += 1
                else:
                    print('.', end='')
                    countll += 1
                if countll == W:
                    print()
                    break
        elif countl % 2 == 1:
            countll = 0
            while True:
                if countll % 2 == 0:
                    print('.', end='')
                    countll += 1
                else:
                    print('#', end='')
                    countll += 1
                if countll == W:
                    print()
                    break
        countl += 1
    print()

Fixed submission:
while True:                                                                                      
    H, W = map(int, input().split())
    if H == 0 and W == 0:
        break
    countl = 0
 
    while True:
        if W == 0:
            break
        if countl == H:
            break
        elif countl % 2 == 0:
            countll = 0
            while True:
                if countll % 2 == 0:
                    print('#', end='')
                    countll += 1
                else:
                    print('.', end='')
                    countll += 1
                if countll == W:
                    print()
                    break
        elif countl % 2 == 1:
            countll = 0
            while True:
                if countll % 2 == 0:
                    print('.', end='')
                    countll += 1
                else:
                    print('#', end='')
                    countll += 1
                if countll == W:
                    print()
                    break
        countl += 1
    print()