Python是否具有三元条件运算符? [英] Does Python have a ternary conditional operator?

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，它是在版本2.5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，然后根据布尔值为condition.如果condition评估为True，则对a进行评估并返回，但b被忽略，否则，当对b进行评估并返回但a被忽略时.

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.

这允许发生短路，因为当condition为true时，仅评估a而根本不评估b，但是，当condition为false时，仅评估b而未评估a完全评估过.

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，条件语句是表达式，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

认为条件表达式是在两个值之间切换.当您处于一种价值或另一种"状况时，它非常有用，但是在其他方面却无济于事.

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，则必须使用常规的if 声明，而不是有条件的表达式.

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

请记住，一些Pythonista对此不以为然，

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

• 参数的顺序与许多其他语言(例如C，C ++，Go，Perl，Ruby，Java，Javascript等)中经典的condition ? a : b三元运算符的顺序不同，这可能会导致错误当人们不熟悉Python的令人惊讶"行为时，会使用它(他们可能会颠倒参数顺序).
• 有人认为它笨拙"，因为它违背了正常的思维流程(首先思考条件，然后思考效果).
• 风格上的原因. (尽管'inline if'可能确实非常有用，并且使您的脚本更加简洁，但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)

如果您在记住命令时遇到麻烦，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大声读为x will be 4 if b is greater than 8 otherwise 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
• 是否等效于C的?:"三元运算符?

• Conditional expressions
• Is there an equivalent of C’s "?:" ternary operator?

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