Python有三元条件运算符吗?

问题描述

如果 Python 没有三元条件运算符，是否可以使用其他语言结构来模拟一个?

If Python does not have a ternary conditional operator, is it possible to simulate one using other language constructs?

推荐答案

是的，是 在 2.5 版中添加.表达式语法为:

Yes, it was added in version 2.5. The expression syntax is:

a if condition else b

首先评估 condition，然后根据 a 或 b 中的一个://en.wikipedia.org/wiki/Boolean_data_type" rel="noreferrer" title="布尔数据类型">布尔 条件的值.如果 condition 计算结果为 True，则计算并返回 a 但忽略 b，否则当 >b 被评估并返回，但 a 被忽略.

First condition is evaluated, then exactly one of either a or b is evaluated and returned based on the Boolean value of condition. If condition evaluates to True, then a is evaluated and returned but b is ignored, or else when b is evaluated and returned but a is ignored.

这允许短路，因为当 condition 为真时，仅评估 a 而根本不评估 b，但当 >condition 为 false，仅评估 b 而根本不评估 a.

This allows short-circuiting because when condition is true only a is evaluated and b is not evaluated at all, but when condition is false only b is evaluated and a is not evaluated at all.

例如:

>>> 'true' if True else 'false'
'true'
>>> 'true' if False else 'false'
'false'

请注意，条件是表达式，而不是语句.这意味着您不能在条件表达式中使用赋值语句或pass或其他语句:

Note that conditionals are an expression, not a statement. This means you can't use assignment statements or pass or other statements within a conditional expression:

>>> pass if False else x = 3
  File "<stdin>", line 1
    pass if False else x = 3
          ^
SyntaxError: invalid syntax

但是，您可以使用条件表达式来分配变量，如下所示:

You can, however, use conditional expressions to assign a variable like so:

x = a if True else b

将条件表达式视为在两个值之间切换.当您处于一个值或另一个值"的情况时，它非常有用，但它并没有做太多其他事情.

Think of the conditional expression as switching between two values. It is very useful when you're in a 'one value or another' situation, it but doesn't do much else.

如果需要使用语句，则必须使用普通的if 语句，而不是条件表达式.

If you need to use statements, you have to use a normal if statement instead of a conditional expression.

请记住，一些 Python 爱好者不赞成它有几个原因:

Keep in mind that it's frowned upon by some Pythonistas for several reasons:

• 参数的顺序与经典的条件不同?a : b 三元运算符来自许多其他语言(如 C、C++、Go、Perl、Ruby、Java、Javascript 等)，当人们不熟悉 Python 的惊奇"时，可能会导致 bug.行为使用它(他们可能会颠倒参数顺序).
• 有些人认为它笨拙"，因为它与正常的思维流程背道而驰(先考虑条件，然后再考虑效果).
• 风格原因.(虽然 'inline if' 可以真正 有用，并使您的脚本更简洁，但它确实使您的代码复杂化)

• The order of the arguments is different from those of the classic condition ? a : b ternary operator from many other languages (such as C, C++, Go, Perl, Ruby, Java, Javascript, etc.), which may lead to bugs when people unfamiliar with Python's "surprising" behaviour use it (they may reverse the argument order).
• Some find it "unwieldy", since it goes contrary to the normal flow of thought (thinking of the condition first and then the effects).
• Stylistic reasons. (Although the 'inline if' can be really useful, and make your script more concise, it really does complicate your code)

如果您无法记住顺序，请记住，当您大声朗读时，您(几乎)说出了您的意思.例如，x = 4 if b >;8 else 9 被大声读出，因为 如果 b 大于 8，x 将为 4，否则为 9.

If you're having trouble remembering the order, then remember that when read aloud, you (almost) say what you mean. For example, x = 4 if b > 8 else 9 is read aloud as x will be 4 if b is greater than 8 otherwise 9.

官方文档:

• 条件表达式
• 是否有等价于 C 的?:"三元运算符?

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