关于Python while语句的Else子句 [英] Else clause on Python while statement

问题描述

我注意到以下代码在Python中是合法的。我的问题是为什么？是否有特定原因？

I've noticed the following code is legal in Python. My question is why? Is there a specific reason?

n = 5
while n != 0:
    print n
    n -= 1
else:
    print "what the..."

推荐答案

else 子句仅在而条件成为时执行假。如果 break 退出循环，或者引发异常，则不会执行该异常。

The else clause is only executed when your while condition becomes false. If you break out of the loop, or if an exception is raised, it won't be executed.

考虑它的一种方法是作为条件的if / else结构：

One way to think about it is as an if/else construct with respect to the condition:

if condition:
    handle_true()
else:
    handle_false()

类似于循环结构：

while condition:
    handle_true()
else:
    # condition is false now, handle and go on with the rest of the program
    handle_false()

一个例子可能是：

while value < threshold:
    if not process_acceptable_value(value):
        # something went wrong, exit the loop; don't pass go, don't collect 200
        break
    value = update(value)
else:
    # value >= threshold; pass go, collect 200
    handle_threshold_reached()

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