请允许我举例说明为什么要使用else子句。但是:

我的观点在Leo的回答中得到了更好的解释 我使用for-而不是while-循环，但else工作类似(除非遇到break，否则执行) 有更好的方法可以做到这一点(例如，将其包装到函数中或引发异常)

打破多级循环

它是这样工作的:外部循环在结尾有一个break，所以它只会执行一次。但是，如果内部循环完成(没有找到除数)，那么它将到达else语句，并且永远不会到达外部断点。这样，内部循环中的中断将跳出两个循环，而不仅仅是一个循环。

for k in [2, 3, 5, 7, 11, 13, 17, 25]:
    for m in range(2, 10):
        if k == m:
            continue
        print 'trying %s %% %s' % (k, m)
        if k % m == 0:
            print 'found a divisor: %d %% %d; breaking out of loop' % (k, m)
            break
    else:
        continue
    print 'breaking another level of loop'
    break
else:
    print 'no divisor could be found!'

当while-condition的值为false时，else子句将被执行。

从文档中可以看到:

The while statement is used for repeated execution as long as an expression is true: while_stmt ::= "while" expression ":" suite ["else" ":" suite] This repeatedly tests the expression and, if it is true, executes the first suite; if the expression is false (which may be the first time it is tested) the suite of the else clause, if present, is executed and the loop terminates. A break statement executed in the first suite terminates the loop without executing the else clause’s suite. A continue statement executed in the first suite skips the rest of the suite and goes back to testing the expression.

如果while循环没有中断，则执行Else。

我有点喜欢用“跑步者”来比喻。

“else”就像越过终点线，与你是从赛道的起点还是终点出发无关。"else"只有当你在两者之间的某个地方中断时才不会执行。

runner_at = 0 # or 10 makes no difference, if unlucky_sector is not 0-10
unlucky_sector = 6
while runner_at < 10:
    print("Runner at: ", runner_at)
    if runner_at == unlucky_sector:
        print("Runner fell and broke his foot. Will not reach finish.")
        break
    runner_at += 1
else:
    print("Runner has finished the race!") # Not executed if runner broke his foot.

主要的用例是使用这种打破嵌套循环，或者如果你想只在循环没有在某处中断时运行某些语句(认为中断是一种不寻常的情况)。

例如，下面是一个关于如何在不使用变量或try/catch的情况下跳出内部循环的机制:

for i in [1,2,3]:
    for j in ['a', 'unlucky', 'c']:
        print(i, j)
        if j == 'unlucky':
            break
    else: 
        continue  # Only executed if inner loop didn't break.
    break         # This is only reached if inner loop 'breaked' out since continue didn't run. 

print("Finished")
# 1 a
# 1 b
# Finished

else子句只在while条件为false时执行。

下面是一些例子:

例1:初始条件为false，因此执行else-clause。

i = 99999999

while i < 5:
    print(i)
    i += 1
else:
    print('this')

输出:

this

例2:while-condition i < 5永远不会变成false，因为i == 3打破了循环，所以else-clause没有执行。

i = 0

while i < 5:
    print(i)
    if i == 3:
        break
    i += 1
else:
    print('this')

输出:

0
1
2
3

例3:while-condition i < 5在i为5时变为false，因此执行else-clause。

i = 0

while i < 5:
    print(i)
    i += 1
else:
    print('this')

输出:

0
1
2
3
4
this

As far as I know the main reason for adding else to loops in any language is in cases when the iterator is not on in your control. Imagine the iterator is on a server and you just give it a signal to fetch the next 100 records of data. You want the loop to go on as long as the length of the data received is 100. If it is less, you need it to go one more times and then end it. There are many other situations where you have no control over the last iteration. Having the option to add an else in these cases makes everything much easier.

我知道这是个老问题，但是…

正如Raymond Hettinger所说，应该调用while/no_break而不是while/else。 如果你看一下这个片段，我发现它很容易理解。

n = 5
while n > 0:
    print n
    n -= 1
    if n == 2:
        break
if n == 0:
    print n

现在，我们不用在while循环后检查condition，而是用else替换它，然后去掉那个检查。

n = 5
while n > 0:
    print n
    n -= 1
    if n == 2:
        break
else:  # read it as "no_break"
    print n

我总是把它读成while/no_break来理解代码，这种语法对我来说更有意义。