单个下划线"_"的目的是什么? Python中的变量? [英] What is the purpose of the single underscore "_" variable in Python?

问题描述

这段代码中for之后的_是什么意思?

What is the meaning of _ after for in this code?

if tbh.bag:
   n = 0
   for _ in tbh.bag.atom_set():
      n += 1

推荐答案

_在Python中有4种主要的常规用法:

_ has 4 main conventional uses in Python:

1. 以交互方式保存最后执行的表达式(/语句)的结果 口译会议.此先例是由标准CPython设定的 口译员，其他口译员也纷纷效仿
2. 有关在i18n中进行翻译查找的信息(请参阅 gettext 例如文档)，如代码所示: raise forms.ValidationError(_("Please enter a correct username"))
3. 作为通用可丢弃"变量名来表示该部分 函数结果的故意被忽略(从概念上讲，它被丢弃了.)，如以下代码所示: label, has_label, _ = text.partition(':').
4. 作为函数定义的一部分(使用def或lambda)，其中 签名是固定的(例如通过回调或父类API)，但是 这个特定的功能实现不需要所有 参数，如以下代码所示:callback = lambda _: True

1. To hold the result of the last executed expression(/statement) in an interactive interpreter session. This precedent was set by the standard CPython interpreter, and other interpreters have followed suit
2. For translation lookup in i18n (see the gettext documentation for example), as in code like: raise forms.ValidationError(_("Please enter a correct username"))
3. As a general purpose "throwaway" variable name to indicate that part of a function result is being deliberately ignored (Conceptually, it is being discarded.), as in code like: label, has_label, _ = text.partition(':').
4. As part of a function definition (using either def or lambda), where the signature is fixed (e.g. by a callback or parent class API), but this particular function implementation doesn't need all of the parameters, as in code like: callback = lambda _: True

(很长一段时间以来，此答案只列出了前三个用例，但经常出现第四个用例，如

(For a long time this answer only listed the first three use cases, but the fourth case came up often enough, as noted here, to be worth listing explicitly)

后面的丢弃变量或参数名称"用例可能与翻译查找用例冲突，因此有必要避免在也将其用于i18n转换的任何代码块中将_用作丢弃变量.出于这个原因，人们更喜欢使用双下划线__作为他们的一次性变量).

The latter "throwaway variable or parameter name" uses cases can conflict with the translation lookup use case, so it is necessary to avoid using _ as a throwaway variable in any code block that also uses it for i18n translation (many folks prefer a double-underscore, __, as their throwaway variable for exactly this reason).

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