The easiest way I found to 'get' what the for/else did, and more importantly, when to use it, was to concentrate on where the break statement jumps to. The For/else construct is a single block. The break jumps out of the block, and so jumps 'over' the else clause. If the contents of the else clause simply followed the for clause, it would never be jumped over, and so the equivalent logic would have to be provided by putting it in an if. This has been said before, but not quite in these words, so it may help somebody else. Try running the following code fragment. I'm wholeheartedly in favour of the 'no break' comment for clarity.

for a in range(3):
    print(a)
    if a==4: # change value to force break or not
        break
else: #no break  +10 for whoever thought of this decoration
    print('for completed OK')

print('statement after for loop')

编辑-我注意到这个问题仍然在运行

第二个更好的想法……

“没有休息”的评论是负面的。要理解一个正的断言要容易得多，那就是for可迭代对象已经用完了。

for a in range(3):
    print(a)
    if a==4: # change value to force break or not
        print('ending for loop with a break')
        break
else: # for iterable exhausted  
    print('ending for loop as iterable exhausted')

print('for loop ended one way or another')

这也强化了这种解释

if iterable_supplies_a_value:
    run_the_for_with_that_value
else:
    do_something_else

for i in range(3):
    print(i)

    if i == 2:
        print("Too big - I'm giving up!")
        break;
else:
    print("Completed successfully")

"else"在这里非常简单，意思是

1、“if for从句是完整的”

for i in range(3):
    print(i)

    if i == 2:
        print("Too big - I'm giving up!")
        break;
if "for clause is completed":
    print("Completed successfully")

写“for从句已完成”这样长的语句是很笨拙的，所以他们引入了“else”。

否则这里本质上是一个如果。

2、然而，How about for从句根本没有出现

In [331]: for i in range(0):
     ...:     print(i)
     ...: 
     ...:     if i == 9:
     ...:         print("Too big - I'm giving up!")
     ...:         break
     ...: else:
     ...:     print("Completed successfully")
     ...:     
Completed successfully

所以这完全是逻辑组合:

if "for clause is completed" or "not run at all":
     do else stuff

或者这样说:

if "for clause is not partially run":
    do else stuff

或者这样:

if "for clause not encounter a break":
    do else stuff

我把它读成这样:“当可迭代对象完全耗尽时，在完成for语句后，执行即将继续执行下一个语句，else子句将被执行。”因此，当迭代被break打断时，这将不会执行。

很好的回答是:

这就解释了历史，而且 这就赋予了 引用以简化您的翻译/理解。

我在这里的注释来自Donald Knuth曾经说过(抱歉找不到参考)，有一个结构，其中while-else与if-else无法区分，即(在Python中):

x = 2
while x > 3:
    print("foo")
    break
else:
    print("boo")

具有相同的流程(不包括低级别差异):

x = 2
if x > 3:
    print("foo")
else:
    print("boo")

关键是if-else可以被认为是while-else的语法糖，它在if块的末尾有隐含的break。相反的含义是，while循环是if的扩展，更常见(它只是重复/循环的条件检查)，因为if经常在while之前教授。然而，这不是真的，因为这将意味着else块在while-else每次当condition为false时执行。

为了便于理解，可以这样想:

没有break, return等，循环只在condition不再为真时结束，在这种情况下else块也将执行一次。在Python for的情况下，你必须考虑c风格的for循环(带条件)或将它们转换为while。

另注:

循环中的过早中断、返回等使得condition不可能变为false，因为当condition为true时，执行跳出了循环，并且它永远不会再回来检查它。

我认为文档对else有很好的解释，继续

[…当循环因列表耗尽而终止(使用for)或当条件变为false(使用while)时执行，但当循环由break语句终止时不执行。”

来源:Python 2文档:控制流教程

一个循环的else分支只执行一次，不管这个循环是否进入它的循环体，除非循环体已经进入但没有结束。也就是说，在循环中会遇到break或return语句。

my_list = []
for i in my_list:
    print(i, end=',')
else:
    print('loop did not enter')

##################################    

for i in range(1,6,1):
    print(i, end=',')
else:
    print('loop completed successfully:', i)

##################################    

for i in range(1,6,1):
    if i == 3:
        print('loop did not finish:', i)
        break
    print(i, end=',')
else:
    print('else:', i)

输出:

loop did not enter
1,2,3,4,5,loop completed successfully: 5
1,2,loop did not finish: 3

while-else也是一样的。

import random
random.seed(8)

i = 100
while i < 90:
    print(i, end=',')
    i = random.randint(0,100)
else:
    print('loop did not enter:', i)

##################################    

i = 25
while i < 90:
    print(i, end=',')
    i = random.randint(0,100)
else:
    print('loop completed successfully:', i)

##################################    

i = 25
while i < 90:
    if i % 10 == 0:
        print('loop did not finish:', i)
        break
    print(i, end=',')
    i = random.randint(0,100)
else:
    print('else:', i)

输出:

loop did not enter: 100
25,29,47,48,16,24,loop completed successfully: 90
25,5,loop did not finish: 10