“if”，“elif”和“elif”。链与普通的“如果”链 [英] "if", and "elif" chain versus a plain "if" chain

问题描述

我想知道，为什么在你可以这么做时需要使用 elif ？

I was wondering, why is using elif necessary when you could just do this?

if True:
    ...
if False:
    ...
...

推荐答案

当你想确保这个时，你会使用 elif 只挑选一个分支：

You'd use elif when you want to ensure that only one branch is picked:

foo = 'bar'
spam = 'eggs'

if foo == 'bar':
    # do this
elif spam == 'eggs':
    # won't do this.

与此相比：

foo = 'bar'
spam = 'eggs'

if foo == 'bar':
    # do this
if spam == 'eggs':
    # *and* do this.

只有 if 语句，选项是非独家。

With just if statements, the options are not exclusive.

当 if 分支更改程序状态时，这也适用 elif 测试也可能是真的：

This also applies when the if branch changes the program state such that the elif test might be true too:

foo = 'bar'

if foo == 'bar':
    # do this
    foo = 'spam'
elif foo == 'spam':
    # this is skipped, even if foo == 'spam' is now true
    foo = 'ham'

此处 foo 将设置为'spam'。

foo = 'bar'

if foo == 'bar':
    # do this
    foo = 'spam'
if foo == 'spam':
    # this is executed when foo == 'bar' as well, as 
    # the previous if statement changed it to 'spam'.
    foo = 'ham'

现在 foo 设置为'spam'，然后设置为'ham'。

Now foo is set to 'spam', then to 'ham'.

从技术上讲， elif 是（复合） if 语句的一部分; Python在一系列中选择第一个测试，如果 / elif 测试为true的分支，如果没有，则为 else 分支（如果存在）。使用单独的 if 语句启动新选择，独立于之前的 if 复合语句。

Technically speaking, elif is part of the (compound) if statement; Python picks the first test in a series of if / elif branches that tests as true, or the else branch (if present) if none are true. Using a separate if statement starts a new selection, independent of the previous if compound statement.

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