underscores _ in Python

Understanding the underscore( _ ) of Python :

The underscore ( _ ) is special in Python.

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.

When used in interpreter :

• The python interpreter stores the last expression value to the special variable called ‘_’.

Example:

>>> x=10

>>> x

10

>>> _

10

>>> _*2

20

>>> _

20

>>> _+3

23

>>> _

23

>>> y=_/3

>>> y

7.666666666666667


For Ignoring the values:

• 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.

Example:

Ignore a value when unpacking:

>>> x, _, y = (1, 2, 3)

>>> x

1

>>> y

3

>>> 

Ignore the multiple values:

• Note:which is available in only Python 3.x.

>>> x, *_, y = (1, 2, 3, 4, 5)

>>> x

1

>>> y

5

>>> 

Ignore the index:

for _ in range(10):

print("hai")

hai

hai

hai

hai

hai

hai

hai

hai

hai

hai

def do_something( ):

print("hello")

for _ in range(10):

do_something()

hello

hello

hello

hello

hello

hello

hello

hello

hello

hello

Ignore a value of specific location:

>>> x=(5,10,15,20,25)

>>> for _,val in x:

do_something()



Give special meanings to name of variables and functions:

• The underscore may be most used in ‘naming’.

• The PEP8 which is Python convention guideline introduces the following 4 naming cases.

single underscore( _ ):

• leading underscore( _ )

example:

_variablename or _functionname or _methodname or _classname

• This convention is used for declaring protected variables, functions, methods and classes in a module.

• tailing underscore( _ )

example:

variablename_ or functionname_ or methodname_ or classname_

• This convention could be used for avoiding conflict with Python keywords or built-ins.

Tkinter.Toplevel(master, class_='ClassName')
# Avoid conflict with 'class' keyword

list_ = List.objects.get(1)
# Avoid conflict with 'list' built-in type


double underscore( _ _ ):

• leading double underscore(_ _)

example:
 _ _varaiblename or _ _functionname or _ _methodname or _ _classname

• double underscore will mangle the attribute names of a class to avoide the conflicts of attribute names between classes.

• (so-called “mangling” that means that the compiler or interpreter modify the variables or function names with some rules, not use as it is). 

• The mangling rule of Python is adding the “_ClassName” to front of attribute names are declared with double underscore. 

• That is, if you write method named “__method” in a class, the name will be mangled in “_ClassName__method” form.

• leading and tailing double underscores(_ _)

example:
_ _variablename_ _ or _ _functionname_ _ or _ _classname_ _ or __methodname__

• This convention is used for special variables or methods (so-called “magic method”) such as   _ _init_ _  ,  _ _len__  ,          _ _new_ _ ,_ _del_ _,......etc.

• These methods provides special syntactic features or does special things.

example:
class A:
    def __init__(self, a):
          self.a = a
    def __custom__(self):
               pass


As Internationalization(i18n)/Localization(l10 n) functions :

• The built-in library gettext which is for i18n/l10n uses this convention, and Django which is Python web framework supports i18n/l10n also introduces and uses this convention.

# see official docs : https://docs.python.org/3/library/gettext.html import gettext
gettext.bindtextdomain('myapplication','/path/to/my/language /directory') gettext.textdomain('myapplication') _ = gettext.gettext
# ...
print(_('This is a translatable string.'))



To separate the digits of number literal value:

• This feature was added in Python 3.6. It is used for separating digits of numbers using underscore for readability.

>>> x=1_00

>>> x

100

>>> y="siva_krishna"

>>> y

'siva_krishna'

>>> z=0b_101

>>> bin(z)

'0b101'
>>>