为什么“else"的目的是“for"后面的子句或“同时"环形? [英] Why is the purpose of the "else" clause following a "for" or "while" loop?

问题描述

我是 Python 初学者.我发现 for-else 和 while-else 中的 else 完全是不必要.因为for 和while 最终会运行到else，我们可以使用通常的行来代替.

I am a Python beginner. I find that the else in for-else and while-else is completely unnecessary. Because for and while will finally run to else, and we can use the usual lines instead.

例如:

for i in range(1, 5):
    print i
else:
    print 'over'

和

for i in range(1, 5):
    print i
print 'over'

都一样.

那么为什么Python在for-else和while-else中有else代码>?

So why does Python have else in for-else and while-else?

推荐答案

你对 for/else 的语义理解有误.else 子句仅在循环完成时运行，例如，如果没有遇到 break 语句.

You are wrong about the semantics of for/else. The else clause runs only if the loop completed, for example, if a break statement wasn't encountered.

典型的 for/else 循环如下所示:

The typical for/else loop looks like this:

for x in seq:
    if cond(x):
        break
else:
    print "Didn't find an x I liked!"

将else"视为与循环体中的所有if"配对.您的示例是相同的，但混合了break"语句，它们不是.

Think of the "else" as pairing with all of the "if's" in the loop body. Your samples are the same, but with "break" statements in the mix, they are not.

对同一想法的更长描述:http://nedbatchelder.com/blog/201110/forelse.html

A longer description of the same idea: http://nedbatchelder.com/blog/201110/forelse.html

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