在由IPython内核提供支持的Jupyter笔记本中重置下划线(`_`)变量 [英] Reset underscore (`_`) variable in Jupyter notebook powered by IPython kernel

问题描述

编辑:已在 GitHub .我将问题留在这里，以防其他人发现问题(我无法).

Edit: The issue has been reported in GitHub. I'm leaving the question here in case it helps other people find the issue (I was not able to).

在Jupyter笔记本中工作时，为了方便起见，我经常使用_变量(它返回最新代码执行的输出).但是，当_用作未使用变量的占位符(Python中的典型用例)时，它将破坏第一个用例.

I often use the _ variable for convenience when working in a Jupyter notebook (it returns the output of the latest code execution). However, when _ is used to as a placeholder for an unused variable (a typical use case in Python), it breaks the first use case.

请注意，这在IPython控制台中可以正常工作.在下面，_在用作循环中的未使用占位符后再次保留了最新的返回值.

Note that this works as expected in an IPython console. Below, _ again holds the latest returned value after being used as an unused placeholder in the loop.

In [1]: 'value'
Out[1]: 'value'

In [2]: _
Out[2]: 'value'

In [3]: for _ in range(2):
   ...:     print('hello')
   ...:     
hello
hello

In [4]: _
Out[4]: 1

In [5]: 'value'
Out[5]: 'value'

In [6]: _
Out[6]: 'value'

但是，在Jupyter笔记本中运行相同的代码后，无论最新输出是什么，_都将永久保留1(循环中的最后一个值).如果我尝试del _，则_将不再是可访问的变量.

However, after running the same code in a Jupyter notebook, _ will forever hold 1 (the last value from the loop), no matter what the latest output is. If I try to del _, then _ will no longer be an accessible variable.

简而言之，Python中_变量的两种用法在Jupyter笔记本中发生冲突，但在IPython控制台中不是.这只是一个不便，但我很想知道如何解决它-或为什么会这样.

In short, the two uses of the _ variable in Python clash in a Jupyter notebook, but not in an IPython console. It's only an inconvenience, but I would be curious to know how to solve it - or why it is.

修改:

$ python --version
Python 3.6.3 :: Anaconda, Inc.
$ ipython --version
6.5.0
$ jupyter notebook --version
5.6.0

推荐答案

根据IPython源代码进行 ../lib/site-packages/IPython/core/displayhook.py 197 update_user_ns

Accooding to IPython source code ../lib/site-packages/IPython/core/displayhook.py 197 update_user_ns

            update_unders = True
            for unders in ['_'*i for i in range(1,4)]:
                if not unders in self.shell.user_ns:
                    continue
                if getattr(self, unders) is not self.shell.user_ns.get(unders):
                    update_unders = False

            self.___ = self.__
            self.__ = self._
            self._ = result

要恢复下划线变量功能，只需在ipython repl中运行此代码

To recover underscore variable functionality,just run this code in ipython repl

out_len=len(Out)
for index,n in enumerate(Out):
    if index==out_len-1:   _=Out[n]
    if index==out_len-2:  __=Out[n]
    if index==out_len-3: ___=Out[n]
    if index==out_len-4:____=Out[n]

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