Python 中的 _ 有什么作用? [英] What does _ in Python do?

问题描述

我在某个地方看到了 Python 中使用的 _ 字符，例如:

print _

有人可以帮我解释一下它的作用吗?

解决方案

在交互式解释器中，_ 总是指最后输出的值:

<预><代码>>>>1 + 12>>>打印 _2>>>2 + 24>>>打印 _4>>>

在普通的 Python¹ 代码中，_ 只是一个典型的名字.您可以像其他任何人一样分配给它:

_ = 3打印 _# 输出:3

虽然我不建议实际这样做，因为 _ 是一个糟糕的名字.此外，按照惯例，它用于表示一个名称，它只是一个占位符.一个例子是:

a, _, b = [1, 2, 3]

使用 _ 表示我们对 2 不感兴趣.另一个例子是:

for _ in range(10):功能()

这意味着我们没有在循环内使用 counter 变量.相反，我们只希望 Python 调用 function 十次，并且需要 _ 具有有效的语法.

<小时>

¹Python"是指 CPython，它是该语言的标准风格.其他实现可能会选择以不同的方式做事.例如，IPython 有关于仅下划线名称的说法:

<块引用>

以下 GLOBAL 变量始终存在(所以不要覆盖他们！):

[_](单下划线):存储之前的输出，就像 Python 的默认解释器.[__](两个下划线):下一个上一个.[___](三个下划线):下一个下一个上一个.

来源:http://ipython.org/ipython-doc/rel-0.9.1/html/interactive/reference.html#output-caching-system

I saw somewhere about the _ character being used in Python like:

print _

Can somebody help me explain what it does?

解决方案

In the interactive interpreter, _ always refers to the last outputed value:

>>> 1 + 1
2
>>> print _
2
>>> 2 + 2
4
>>> print _
4
>>>

In normal Python¹ code however, _ is just a typical name. You can assign to it as you would any other:

_ = 3
print _
# Output: 3

Although I wouldn't recommend actually doing this because _ is a terrible name. Also, it is used by convention to mean a name that is simply a placeholder. An example would be:

a, _, b = [1, 2, 3]

which uses _ to mean that we are not interested in the 2. Another example is:

for _ in range(10):
    function()

which means that we are not using the counter variable inside the loop. Instead, we only want Python to call function ten times and need the _ to have valid syntax.

---

¹By "Python", I mean CPython, which is the standard flavor of the language. Other implementations may choose to do things differently. IPython for example has this to say about underscore-only names:

The following GLOBAL variables always exist (so don’t overwrite them!):

[_] (a single underscore) : stores previous output, like Python’s default interpreter.
[__] (two underscores): next previous.
[___] (three underscores): next-next previous.

Source: http://ipython.org/ipython-doc/rel-0.9.1/html/interactive/reference.html#output-caching-system

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