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

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。

一个常见的构造是运行一个循环，直到找到一些东西，然后跳出循环。问题是，如果我跳出循环或循环结束，我需要确定发生了哪种情况。一种方法是创建一个标志或存储变量，让我进行第二次测试，以查看循环是如何退出的。

例如，假设我需要在列表中搜索并处理每个项，直到找到标志项，然后停止处理。如果缺少标志项，则需要引发异常。

使用Python进行…你的其他构念

for i in mylist:
    if i == theflag:
        break
    process(i)
else:
    raise ValueError("List argument missing terminal flag.")

将此方法与不使用此语法糖的方法进行比较:

flagfound = False
for i in mylist:
    if i == theflag:
        flagfound = True
        break
    process(i)

if not flagfound:
    raise ValueError("List argument missing terminal flag.")

在第一种情况下，raise与它所使用的for循环紧密绑定。第二种情况是，绑定不那么牢固，在维护过程中可能会出现错误。

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。

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

因为他们不想给这门语言引入一个新的关键字。每一个都窃取一个标识符并导致向后兼容问题，所以通常是最后的手段。