给定以下代码(不起作用):

while True:
    # Snip: print out current state
    while True:
        ok = get_input("Is this ok? (y/n)")
        if ok.lower() == "y": break 2 # This doesn't work :(
        if ok.lower() == "n": break

    # Do more processing with menus and stuff

有办法解决这个问题吗?还是我必须先检查一次,跳出输入循环,然后再检查一次,如果用户满意,再跳出外部循环?


当前回答

我的第一反应是将嵌套循环重构为一个函数,并使用return来跳出。

其他回答

我想提醒你,Python中的函数可以在代码中间创建,并且可以透明地访问周围的变量以进行读取,也可以通过非局部或全局声明进行写入。

所以你可以使用一个函数作为“易碎的控制结构”,定义一个你想要返回的地方:

def is_prime(number):

    foo = bar = number

    def return_here():
        nonlocal foo, bar
        init_bar = bar
        while foo > 0:
            bar = init_bar
            while bar >= foo:
                if foo*bar == number:
                    return
                bar -= 1
            foo -= 1

    return_here()

    if foo == 1:
        print(number, 'is prime')
    else:
        print(number, '=', bar, '*', foo)

>>> is_prime(67)
67 is prime
>>> is_prime(117)
117 = 13 * 9
>>> is_prime(16)
16 = 4 * 4

要跳出多个嵌套循环,而不需要重构为函数,可以使用带有内置StopIteration异常的“模拟goto语句”:

try:
    for outer in range(100):
        for inner in range(100):
            if break_early():
                raise StopIteration

except StopIteration: pass

请参阅使用goto语句打破嵌套循环的讨论。

# this version uses a level counter to choose how far to break out

break_levels = 0
while True:
    # snip: print out current state
    while True:
        ok = get_input("Is this ok? (y/n)")
        if ok == "y" or ok == "Y":
            break_levels = 1        # how far nested, excluding this break
            break
        if ok == "n" or ok == "N":
            break                   # normal break
    if break_levels:
        break_levels -= 1
        break                       # pop another level
if break_levels:
    break_levels -= 1
    break

# ...and so on

由于这个问题已经成为进入特定循环的标准问题,我想用Exception给出我的答案。

虽然在多循环构造中不存在名为“循环中断”的标签,但我们可以使用用户定义异常来中断到我们选择的特定循环。考虑下面的例子,让我们在6进制编号系统中打印所有最多4位的数字:

class BreakLoop(Exception):
    def __init__(self, counter):
        Exception.__init__(self, 'Exception 1')
        self.counter = counter

for counter1 in range(6):   # Make it 1000
    try:
        thousand = counter1 * 1000
        for counter2 in range(6):  # Make it 100
            try:
                hundred = counter2 * 100
                for counter3 in range(6): # Make it 10
                    try:
                        ten = counter3 * 10
                        for counter4 in range(6):
                            try:
                                unit = counter4
                                value = thousand + hundred + ten + unit
                                if unit == 4 :
                                    raise BreakLoop(4) # Don't break from loop
                                if ten == 30: 
                                    raise BreakLoop(3) # Break into loop 3
                                if hundred == 500:
                                    raise BreakLoop(2) # Break into loop 2
                                if thousand == 2000:
                                    raise BreakLoop(1) # Break into loop 1

                                print('{:04d}'.format(value))
                            except BreakLoop as bl:
                                if bl.counter != 4:
                                    raise bl
                    except BreakLoop as bl:
                        if bl.counter != 3:
                            raise bl
            except BreakLoop as bl:
                if bl.counter != 2:
                    raise bl
    except BreakLoop as bl:
        pass

当我们打印输出时,我们永远不会得到任何单位位是4的值。在这种情况下,在同一个循环中引发BreakLoop(4)并捕获时,我们不会中断任何循环。类似地,当十位有3时,我们使用BreakLoop(3)进入第三个循环。当百位是5时,我们使用BreakLoop(2)进入第二个循环,当千位是2时,我们使用BreakLoop(1)进入第一个循环。

简而言之,在内部循环中引发异常(内置或用户定义),并在循环中从您想恢复控件的位置捕获它。如果想从所有循环中中断,可以在所有循环之外捕获异常。(我没有举例说明)。

Break for外层和内部while循环:

while True:
    while True:
        print('Breaks inner "while" loop')
        break # Here
    print('Breaks outer "while" loop')
    break # Here

或者,用if语句中断外部和内部while循环:

while True:
    while True:
        if True:
            print('Breaks inner "while" loop')
            break # Here
    print('Breaks outer "while" loop')
    break # Here

输出:

Breaks inner "while" loop
Breaks outer "while" loop

Break for outer和inner for循环:

for _ in iter(int, 1):
    for _ in iter(int, 1):
        print('Breaks inner "for" loop')
        break # Here
    print('Breaks outer "for" loop')
    break # Here

或者,用if语句打破外部和内部for循环:

for _ in iter(int, 1):
    for _ in iter(int, 1):
        if True:
            print('Breaks inner "for" loop')
            break # Here
    print('Breaks outer "for" loop')
    break # Here

输出:

Breaks inner "for" loop
Breaks outer "for" loop