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构造模拟一个?

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，它是 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 condition 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，只计算 a 而根本不计算 b，但是当 condition 为假，仅对 b 求值，而 a 根本不求值.

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，条件是一个表达式，而不是一个语句.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

将条件表达式视为两个值之间的切换.当您处于一种价值或另一种价值"的情况下时，它非常有用，但不会做太多其他事情.

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 Pythonistas 不赞成这样做的原因有几个:

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 来自许多其他语言(如 C、C++、Go、Perl、Ruby、Java、Javascript 等)的三元运算符，当人们不熟悉 Python 的惊喜"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"，因为它违背了正常的思维流程(先考虑条件，然后考虑效果).
• 风格原因.(尽管内联 if"可以真的有用，并使您的脚本更加简洁，但它确实会使您的代码复杂化)

• 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)

如果您在记住顺序时遇到困难，请记住，当您大声朗读时，您(几乎)说出了您的意思.例如，x = 4 如果 b >8 else 9 被朗读为 x 为 4 如果 b 大于 8 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
• 是否有等效于 C 的?:"三元运算符?

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