What does _ do in Python?
In the interactive interpreter, it _
always refers to the last output value:
>>> 1 + 1
2
>>> print _
2
>>> 2 + 2
4
>>> print _
4
>>>
However, in normal Python code, 1_
is a typical name. You can assign it like any other:
_ = 3
print _
# Output: 3
Although I would not recommend doing this, because it _
's a terrible name. Also, it is conventionally used as a name, which is just a placeholder. An example would be:
a, _, b = [1, 2, 3]
which is used _
to denote what we are not interested in 2
. Another example:
for _ in range(10):
function()
which means we don't use the counter variable inside the loop. Instead, we want Python to call function
ten times and need to in _
order to have valid syntax.
1 By "Python" I mean CPython, which is the standard flavor of the language. Other implementations can do different things. IPython, for example, has this to say about underscore names:
The following GLOBAL variables always exist (so don't overwrite them):
[_] (a single underscore) : stores previous output, like Python’s default interpreter. [__] (two underscores): next previous. [___] (three underscores): next-next previous.
Source: http://ipython.org/ipython-doc/rel-0.9.1/html/interactive/reference.html#output-caching-system
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It's just a different variable name that is commonly used for three very different things:
In the Python interactive shell, _ is the value of the last expression entered:
>>> 3 + 3
6
>>> _ == 6
True
It is used to indicate that a variable only exists because it should and will not be used:
instance, _ = models.MyModel.objects.get_or_create(name="Whee")
(here get_or_create returns a tuple with two elements, only one of which goes for us to use).
The function used to translate strings (often ugettext) is often renamed locally to _ () so that it takes up as much screen space as possible:
from django.utils.translation import ugettext as _
print(_("This is a translatable string."))
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'_'
is a legitimate symbol for python, like most other programming languages. I think MATLAB is an exception where you cannot use this for any MATLAB file names. I know because I have tried this in the past and it has failed. Don't know if this has been changed since R2014b. The best examples are the python __init__
, __self__
, __str__
, __repr__
etc.
Instead of asking questions and having confusion in your mind, just type it in and see what happens. You won't break anything: D. Open PLEON IDLE and press CTRL +. you will see many native Python variables and functions named _
or even __
. I really liked the @iCodez variable assignment example here :)
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