'... if ... else ...' 表达式的 Python 习语

Python idiom for '... if ... else ...' expression

提问by rukeba

How to write the expression shorter:

如何将表达式写得更短：

return '%.0f' % float_var if float_var else float_var

or

或者

if float_var:
    return formatted_string
else:
    return None

Thanks!

谢谢！

回答by John Feminella

The expression <value> if <condition> else <other_value>is pretty idiomatic already -- certainly more so than the other example, and is probably preferred whenever <value>is simple. This is Python's ternary operator, so if you were looking for something like <condition> ? <value> : <other_value>, that doesn't exist.

这个表达式<value> if <condition> else <other_value>已经非常惯用了——当然比另一个例子更惯用，并且在<value>简单的时候可能是首选。这是 Python 的三元运算符，因此如果您正在寻找类似 的东西<condition> ? <value> : <other_value>，那么它不存在。

If computing <value>or <other_value>takes a few steps, use the longer if: ... else: ...alternative.

如果计算<value>或<other_value>需要几个步骤，请使用更长的if: ... else: ...替代方案。

回答by Dave Kirby

I would use brackets to make the expression more readable:

我会使用括号使表达式更具可读性：

return ('%.0f' % float_var) if float_var else float_var

When I first saw it I read it as

当我第一次看到它时，我把它读为

return '%.0f' % (float_var if float_var else float_var)

which would be silly. I had to try it out to make sure which way it worked.

这将是愚蠢的。我不得不尝试一下，以确定它的工作方式。

BTW Your first example not equivalent to your second example

顺便说一句，你的第一个例子不等同于你的第二个例子

if float_var:
    return formatted_string
else:
    return None

This will always return either a formatted string or None. Your first example if you pass in anything that evaluates to False (False, 0, 0.0, "", [] etc) will return that unchanged, so your return type could be string, boolean, list, int, float etc. This is probably not what you want, especially if 0.0 is a legitimate value for float_var. I would change your code to:

这将始终返回格式化字符串或无。您的第一个示例，如果您传入任何计算结果为 False（假、0、0.0、""、[] 等）的内容，将返回不变，因此您的返回类型可以是字符串、布尔值、列表、整数、浮点数等。这是可能不是您想要的，特别是如果 0.0 是 float_var 的合法值。我会将您的代码更改为：

return ('%.0f' % float_var) if isinstance(float_var, float) else None

alternatively:

或者：

try:
    return "%.0f" % float_var
except TypeError:
    return None

which will work for other integers (and longs) by converting them to float.

通过将它们转换为浮点数，这将适用于其他整数（和长整数）。

回答by Mike Graham

• It is not clear what exactly you want to do.

  1. The most literal interpretation would have it work like this

     >>> float_var = 4.5
     >>> '%.0f' % float_var if float_var else float_var
     '5' # This is a string
     >>> float_var = 0.0
     >>> '%.0f' % float_var if float_var else float_var
     0.0 # This is a float
     

     which I am almost certain you did not want.

  2. I guess you are wanting to check for Nonewith "if float_var"? If so, you always spell it "if foo is not None", not "if foo", the former being clearer and less bug-prone.

     If that iswhat you intended, I suggest you revise your model. Propagating errors by repeatedly returning Noneis a bad thing: it is ugly and bug-prone and non-idiomatic. Use exceptions instead.

• Shorter is not always better. Your snippets are not painfully long or unwieldly. In fact, you will want to make them a little longerif you use them to avoid a potential bug.

  • Someone may suggest abusing the short-circuiting behavior of orfor this. However, this makes code harder to read and doesn't allow you to specify between Noneand other false values, which often leads to bugs.

• 目前还不清楚你到底想做什么。

  1. 最字面的解释会像这样工作

     >>> float_var = 4.5
     >>> '%.0f' % float_var if float_var else float_var
     '5' # This is a string
     >>> float_var = 0.0
     >>> '%.0f' % float_var if float_var else float_var
     0.0 # This is a float
     

     我几乎可以肯定你不想要。

  2. 我猜你想None用“ if float_var”来检查？如果是这样，你总是拼写“ if foo is not None”，而不是“ if foo”，前者更清晰，更不容易出错。

     如果这是您的意图，我建议您修改您的模型。通过重复返回None来传播错误是一件坏事：它丑陋、容易出错且不符合习惯。改用异常。

• 更短并不总是更好。你的代码片段不是很长或很笨拙。事实上，如果你使用它们来避免潜在的错误，你会想要让它们更长一点。

  • 有人可能会建议or为此滥用短路行为。但是，这会使代码更难阅读，并且不允许您在None和 其他假值之间指定，这通常会导致错误。

回答by msw

If you are using are already using v if c else uyou are already using the most readable and efficient ternary operator.

如果您正在使用 are already using，v if c else u那么您已经在使用最易读和最高效的三元运算符。

There are other waysbut they suffer in readability.

还有其他方法，但它们在可读性方面受到影响。

回答by DasIch

float_var and "%.0f" % float_vav

Isn't it awesome?

是不是很棒？