Why does typing _ in the Python interpreter return True?

Question

I am getting very weird interpreter behaviour:

>>> _
True
>>> type(True)
<class 'bool'>
>>> type(_)
<class 'bool'>

I tried this because _ came up as a suggestion in Bpython, but it seems to work in the normal interpreter too. I am using

Python 3.3.3 (v3.3.3:c3896275c0f6, Nov 16 2013, 23:39:35) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin

Can anybody explain why _ is a substitute for True? Is it legacy, similarly to how ";" can be used to end commands, but is not necessary/encouraged?

EDIT: It seems to be random. This does not happen in a new terminal, but once I start working on something _ starts becoming true. What the hell is going on?

I get `NameError: name '_' is not defined`. Close the terminal, start afresh, try again. — thefourtheye, Mar 08 '14 at 13:24
See http://stackoverflow.com/questions/200020/python-get-last-answer — Bach, Mar 08 '14 at 13:27
@thefourtheye That's what I thought too, but it seems to re-appear everytime I get back to work. — Azsgy, Mar 08 '14 at 13:28
Note that if you do `>>> type(True)` and then `>>> type(_)` you get `` as output, and not ``. — Bakuriu, Mar 08 '14 at 13:29
Related: [Assigning a value to \_ in Python/IPython interpreter](http://stackoverflow.com/q/17580289) — Martijn Pieters, Mar 08 '14 at 13:31

score 7 · Accepted Answer · answered Mar 08 '14 at 13:28

_ will be the result of the last evaluated command - at interpreter start up there isn't any so you'll get a NameError... after that, you'll get the previous result... Try opening a new interpreter and doing 2 + 2... you'll see 4 returned, then type _... eg:

>>> _

Traceback (most recent call last):
  File "<pyshell#18>", line 1, in <module>
    _
NameError: name '_' is not defined
>>> 2 + 2
4
>>> _
4

score 4 · Answer 2 · answered Mar 08 '14 at 13:26

4

2 + 1
Out[19]: 3

_ + 3
Out[20]: 6

_ stores the last returned value. Try it out.

answered Mar 08 '14 at 13:26

Stefan van den Akker

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score 4 · Answer 3 · edited May 23 '17 at 10:31

4

_ simply gives you the last result evaluated (in the REPL, not in an ordinary script). This can also mysteriously prevent objects from being deleted immediately.

edited May 23 '17 at 10:31

Community

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answered Mar 08 '14 at 13:26

detly

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score 1 · Answer 4 · answered Mar 08 '14 at 13:29

1

_ in the interactive interperter is usually the last output you received.

>>> 1 + 1
2
>>> _
2

Note it only applies to outputs (printed data won't do).

answered Mar 08 '14 at 13:29

yuvi

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Why does typing _ in the Python interpreter return True?

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