假设有如下代码：

for i in range(10):
    if i == 5:
        print 'found it! i = %s' % i
else:
    print 'not found it ...'

你期望的结果是，当找到5时打印出：

found it! i = 5

实际上打印出来的结果为：

found it! i = 5
not found it ...

显然这不是我们期望的结果。

根据官方文档说法：

When the items are exhausted (which is immediately when the sequence is empty), the suite in 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 continues with the next item, or with the else clause if there was no next item.

https://docs.python.org/2/reference/compound_stmts.html#the-for-statement

大意是说当迭代的对象迭代完并为空时，位于else的子句将执行，而如果在for循环中含有break时则直接终止循环，并不会执行else子句。

所以正确的写法应该为：

'''
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'''
for i in range(10):
    if i == 5:
        print 'found it! i = %s' % i
        break
else:
    print 'not found it ...'

当使用pylint检测代码时会提示Else clause rel="stylesheet">