[FIXED] Reset underscore (`_`) variable in Jupyter notebook powered by IPython kernel

Issue

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).

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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.

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'

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.

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.

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Edit:

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

Solution

Another way is the following:

import builtins
del builtins._
del _
del __
del ___

Just ignore any possible errors. In particular, if you get NameError: name '_' is not defined, you must continue deleting __ etc.

After doing this, it will continue working from the subsequent cell (i.e. this method won't help getting the previous Out[n])

Explanation: in the piece of the code quoted by the other answer (source),

• the code checks if the value of _, __, ___ (note that range(1, 4) excludes 4! So assigning ____ is useless) is either undefined or the same object as the last 3 Out[n]: values.
• if it is, then everything is updated.
• otherwise, nothing is updated.

Thus, if all of _, __, and ___ are undefined, then starting from the next cell, the output will be assigned to _. Importantly, if any one of these are incorrect, it will not work.

The del builtins._ is necessary because otherwise if the user or some library previously did something like

import gettext
gettext.NullTranslations().install()

then builtins._ will be overwritten, and it is necessary to delete it.

Answered By - user202729