What is the meaning of _
after for
in this code?
if tbh.bag:
n = 0
for _ in tbh.bag.atom_set():
n += 1
This question is related to
python
variables
naming-conventions
metasyntactic-variable
It's just a variable name, and it's conventional in python to use _
for throwaway variables. It just indicates that the loop variable isn't actually used.
As far as the Python languages is concerned, _
has no special meaning. It is a valid identifier just like _foo
, foo_
or _f_o_o_
.
Any special meaning of _
is purely by convention. Several cases are common:
A dummy name when a variable is not intended to be used, but a name is required by syntax/semantics.
# iteration disregarding content
sum(1 for _ in some_iterable)
# unpacking disregarding specific elements
head, *_ = values
# function disregarding its argument
def callback(_): return True
Many REPLs/shells store the result of the last top-level expression to builtins._
.
The special identifier
_
is used in the interactive interpreter to store the result of the last evaluation; it is stored in thebuiltins
module. When not in interactive mode,_
has no special meaning and is not defined. [source]
Due to the way names are looked up, unless shadowed by a global or local _
definition the bare _
refers to builtins._
.
>>> 42
42
>>> f'the last answer is {_}'
'the last answer is 42'
>>> _
'the last answer is 42'
>>> _ = 4 # shadow ``builtins._`` with global ``_``
>>> 23
23
>>> _
4
Note: Some shells such as ipython
do not assign to builtins._
but special-case _
.
In the context internationalization and localization, _
is used as an alias for the primary translation function.
Return the localized translation of message, based on the current global domain, language, and locale directory. This function is usually aliased as _() in the local namespace (see examples below).
Underscore _
is considered as "I don't Care" or "Throwaway" variable in Python
The python interpreter stores the last expression value to the special variable called _
.
>>> 10
10
>>> _
10
>>> _ * 3
30
The underscore _
is also used for ignoring the specific values. If you don’t need the specific values or the values are not used, just assign the values to underscore.
Ignore a value when unpacking
x, _, y = (1, 2, 3)
>>> x
1
>>> y
3
Ignore the index
for _ in range(10):
do_something()
There are 5 cases for using the underscore in Python.
For storing the value of last expression in interpreter.
For ignoring the specific values. (so-called “I don’t care”)
To give special meanings and functions to name of variables or functions.
To use as ‘internationalization (i18n)’ or ‘localization (l10n)’ functions.
To separate the digits of number literal value.
Here is a nice article with examples by mingrammer.
Source: Stackoverflow.com