When I input _ or __ in the python shell I get values returned. For instance:
>>> _
2
>>>__
8
What is happening here?
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davejagoda
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tdolydong
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Are you using a "normal" Python shell? – smeso Jan 03 '14 at 15:15
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@BartoszKP: I didn't add thank you, it was in original post. i corrected the spelling, that's all. – Infinite Recursion Jan 03 '14 at 15:18
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2For `_`, you may have look at this answer [_ in python](http://stackoverflow.com/a/3969587/2438678) – Charles0429 Jan 03 '14 at 15:18
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@BartoszKP he didn't add anything in the edit. He corrected a word misspelling. – Skindeep2366 Jan 03 '14 at 15:19
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@Payeli Right, sorry, I've misread it. Anyway - you should've remove it then :) – BartoszKP Jan 03 '14 at 15:20
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when i input `__name__`, i got `'__main__'`.And I think it's global variables. – tdolydong Jan 03 '14 at 15:42
4 Answers
16
If you are using IPython then the following GLOBAL variables always exist:
_(a single underscore): stores previous output, like Python’s default interpreter.__(two underscores): next previous.___(three underscores): next-next previous.
Read more about it from IPython documentation: Output caching system.
Martin
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Do you know where it's defined in python. I don't found the path and the file name. – tdolydong Jan 03 '14 at 15:45
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Theoretically these are just ordinary variable names. By convention, a single underscore is used as a don't care variable. For example, if a function returns a tuple, and you're interested only in one element, a Pythonic way to ignore the other is:
_, x = fun()
In some interpreters _ and __ have special meanings, and store values of previous evaluations.
BartoszKP
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2Worth noting that this is not only (or originally) "pythonic". In some languages which do not allow destructive assignment, `_` is the only variable which can be reassigned, but can never be read. – Hyperboreus Jan 03 '14 at 15:35
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In Python it means what you tell it to mean. Underscores are valid characters in a name. (However, if you are using IPython see Martin's fine answer.)
Python 2.7.5 (default, Aug 25 2013, 00:04:04)
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin
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
>>> _=2
>>> _
2
>>> __
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name '__' is not defined
>>> __=3
>>> __
3
That said, they do have some special semantics. Starting a name with a single underscore doesn't do anything programmatically different, but by convention it tells you the name is intended to be private. But if you start a name with two underscores the interpreter will obfuscate it.
>>> class Bar:
... _=2
... __=3
... _x=2
... __x=3
...
>>> y=Bar()
>>> y._
2
>>> y.__
3
>>> y._x
2
>>> y.__x
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: Bar instance has no attribute '__x'
>>> dir(y)
['_', '_Bar__x', '__', '__doc__', '__module__', '_x']
>>> y._Bar__x
3
2
According to this, in iPython:
_(a single underscore): stores previous output, like Python’s default interpreter.__(two underscores): next previous output.
You can also reference previous inputs and outputs depending on the line they are by:
Input: _iX where X is the input line number
Ouput: _X where X is the output line number
Examples:
_
In [1]: 9 * 3
Out[1]: 27
In [2]: _
Out[2]: 27
__
In [1]: 9 * 3
Out[1]: 27
In [2]: 4 * 8
Out[2]: 32
In [3]: __
Out[3]: 27
_iX
In [1]: x = 10
In [2]: y = 5
In [3]: _i1
Out[3]: u'x = 10'
_X
In [1]: x = 10
In [2]: y = 4
In [3]: x + y
Out[3]: 14
In [4]: _3
Out[4]: 14
lmiguelvargasf
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