Variable assigned to the last executed line?

Question

I just found something weird in the Python interpreter. Let me show you:

$ python
Python 2.7.1+ (r271:86832, Apr 11 2011, 18:13:53) 
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> _
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '_' is not defined
>>> 5 + 4
9
>>> _
9
>>> 'Hello world'
'Hello world'
>>> _
'Hello world'
>>> type(3.5)
<type 'float'>
>>> _
<type 'float'>

You can try this in your interpreter; there are no tricks at work here!

Is the result of the last executed line being assigned to a variable named _?

Does anybody know something about it? Is there any documentation about it? In which situation could it be useful?

exact duplicate http://stackoverflow.com/questions/1538832/is-this-single-underscore-a-built-in-variable-in-python — Johan Lundberg, Jan 31 '12 at 01:38
Well, it's not really a _question_. SO is a _question_ and _answer_ site. If you had asked "Where do I find the implementation of the feature `_` in the Python interpreter?" or something similar, it might have survived. — sarnold, Jan 31 '12 at 01:55
Well, I voted to close as exact duplicate of http://stackoverflow.com/questions/1538832/is-this-single-underscore-a-built-in-variable-in-python so I'm not sure why my name's in the list for 'closed as not a real question' — wim, Jan 31 '12 at 01:58

RanRag · Accepted Answer · 2012-01-31T01:39:32.720

9

Take a look here Reserved identifiers python.

The special identifier _ is used in the interactive interpreter to store the result of the last evaluation; it is stored in the builtin module.

This behavior can be found on haskell's interactive environment ghci also. Here instead of _ use it.

Prelude> 2+2
4
Prelude> it
4

edited Jan 31 '12 at 01:39

answered Jan 31 '12 at 01:32

RanRag

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wim · Answer 2 · 2014-07-27T00:57:21.133

3

It's useful when exploring in the interactive interpreter, when you forgot to assign a name to some returned object, you can grab a reference to it using x = _. Note that in ipython you also have __ for the second-to-last returned, and ___ is the third-to-last.

edited Jul 27 '14 at 00:57

answered Jan 31 '12 at 01:36

wim

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score 0 · Answer 3 · answered Jan 31 '12 at 01:35

0

This isn't a big secret (for example, you can find it mentioned in Code Like a Pythonista) but true, it's not well known. It could be useful when you're doing a lot of work at the command line.

answered Jan 31 '12 at 01:35

jdigital

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score 0 · Answer 4 · edited May 23 '17 at 12:17

0

Per What is the purpose of the single underscore "_" variable in Python?

There are three main uses for _. One is "real" (the usage you discovered), and the other two are conventions.

Interesting... I never knew!

edited May 23 '17 at 12:17

Community

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answered Jan 31 '12 at 01:36

Thomas Kelley

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__ is not only convention. http://docs.python.org/tutorial/classes.html#private-variables – Johan Lundberg Jan 31 '12 at 01:43

Variable assigned to the last executed line?

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