Python while 语句上的 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?有什么具体原因吗?

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 while 条件变为假时执行.如果您break 退出循环，或者如果引发异常，它将不会被执行.

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将其视为与条件相关的 if/else 构造:

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()

一个例子可能是这样的:

An example might be along the lines of:

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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