我理解这种结构是如何运作的:
for i in range(10):
print(i)
if i == 9:
print("Too big - I'm giving up!")
break
else:
print("Completed successfully")
但我不明白为什么在这里使用else作为关键字，因为它表明有问题的代码只在for块未完成时运行，这与它所做的相反!无论我怎么想，我的大脑都不能从for语句无缝地过渡到else语句块。对我来说，continue和continuouswith更有意义(我正试着训练自己这样读它)。
我想知道Python程序员是如何在头脑中(或者大声地，如果你喜欢)阅读这个结构的。也许我遗漏了一些可以使这些代码块更容易破译的东西?
这个问题是关于底层设计决策的，也就是说，为什么能够编写这些代码是有用的。另请参阅Python while语句中的Else子句，了解语法含义的具体问题。
当前回答
简单来说，你可以这样想;
如果它在for循环中遇到break命令，else部分将不会被调用。 如果在for循环中没有遇到break命令，则调用else部分。
换句话说，如果for循环迭代没有被break“打断”，else部分将被调用。
其他回答
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。
你可以这样想， 其他的东西，或者其他的东西，不是在循环中完成的。
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
else语句块中的代码将在for循环未被打破时执行。
for x in xrange(1,5):
if x == 5:
print 'find 5'
break
else:
print 'can not find 5!'
#can not find 5!
来自文档:break和continue语句，以及循环中的else子句
Loop statements may have an else clause; it is executed when the loop terminates through exhaustion of the list (with for) or when the condition becomes false (with while), but not when the loop is terminated by a break statement. This is exemplified by the following loop, which searches for prime numbers: >>> for n in range(2, 10): ... for x in range(2, n): ... if n % x == 0: ... print(n, 'equals', x, '*', n//x) ... break ... else: ... # loop fell through without finding a factor ... print(n, 'is a prime number') ... 2 is a prime number 3 is a prime number 4 equals 2 * 2 5 is a prime number 6 equals 2 * 3 7 is a prime number 8 equals 2 * 4 9 equals 3 * 3 (Yes, this is the correct code. Look closely: the else clause belongs to the for loop, not the if statement.) When used with a loop, the else clause has more in common with the else clause of a try statement than it does that of if statements: a try statement’s else clause runs when no exception occurs, and a loop’s else clause runs when no break occurs. For more on the try statement and exceptions, see Handling Exceptions. The continue statement, also borrowed from C, continues with the next iteration of the loop: >>> for num in range(2, 10): ... if num % 2 == 0: ... print("Found an even number", num) ... continue ... print("Found a number", num) Found an even number 2 Found a number 3 Found an even number 4 Found a number 5 Found an even number 6 Found a number 7 Found an even number 8 Found a number 9
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
