else子句在正常退出一个块时执行，比如击中循环条件或从try块底部掉下去。如果您中断或返回一个块，或引发异常，则不会执行。它不仅适用于while和for循环，还适用于try块。

您通常会在通常会提前退出循环的地方发现它，并且在意想不到/不寻常的情况下运行到循环的末尾。例如，如果你在一个列表中循环寻找一个值:

for value in values:
    if value == 5:
        print "Found it!"
        break
else:
    print "Nowhere to be found. :-("

else:语句当且仅当while循环不再满足它的条件时执行(在您的示例中，当n != 0为假时)。

所以输出是这样的:

5
4
3
2
1
what the...

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.

我的回答将集中在我们什么时候可以使用while/for-else。

乍一看，使用时似乎没有什么不同

while CONDITION:
    EXPRESSIONS
print 'ELSE'
print 'The next statement'

and

while CONDITION:
    EXPRESSIONS
else:
    print 'ELSE'
print 'The next statement'

因为打印'ELSE'语句似乎总是在这两种情况下执行(当while循环结束或不运行时)。

然后，只有当打印'ELSE'语句不被执行时，情况才有所不同。 它是在while下面的代码块中有一个突破

In [17]: i = 0

In [18]: while i < 5:
    print i
    if i == 2:
        break
    i = i +1
else:
    print 'ELSE'
print 'The next statement'
   ....:
0
1
2
The next statement

如与:

In [19]: i = 0

In [20]: while i < 5:
    print i
    if i == 2:
        break
    i = i +1
print 'ELSE'
print 'The next statement'
   ....:
0
1
2
ELSE
The next statement

Return不在此类别中，因为它对上述两种情况具有相同的效果。

异常引发也不会造成差异，因为当它引发时，下一个代码将在异常处理程序中执行(block除外)，else子句或while子句后面的代码将不会被执行。

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。

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