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Python3 for … else …陷阱


举个例子

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 ...'

我们期望的结果是,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。

解答

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。同样的原理适用于while … else循环。

所以正确的写法应该为:

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

假如你使用了pylint检测代码,会提示:

Else clause on loop without a break statement (useless-else-on-loop)

所以养成使用pylint检测代码的习惯还是很有必要的,像这种逻辑错误比较难以发现。但最重要的还是要熟悉文档,甚至源码,追踪溯源,从根本上定位以及解决问题。

其实,在不少书籍中提到,不建议在for和while循环后面使用else语句块!

想想也是,读过一些源代码,也没见过这样的写法呀^_^