Underscore(_)

• Single Underscore (_)

- Stores the value.

>>> 5+4
9

>>> _
9

>>> _+6
15

>>> _
15

 

 

- Ignore the value.

a,_,b=(1,2,3)
print(a,b)

>>>
1 3

lst=['ab','bc','cd']
for _,i in lst:
    print(i)
    
>>>
b
c
d

a,*_,b=(1,2,3,4,5,6,7)
print(a,b)

>>>
1 7

 

• Single Pre Underscore( _variable)

It is used for internal use. When 

# filename : test_1.py

def func():
    print('data')

def _private_func():
    print('7') 
    
#filename : test_2.py

from test_1 import *
func()

>>>
data

_private_func()

>>>
NameError: name '_private_func' is not defined

It I import all the methods from test_1.py to test_2.py, Single Pre Underscore variable(_private_func) doesnt imported.

To avoid the error by importing the module normally as below.

#filename : test_2.py

import test_1 

test_1.func()
test_1._private_func()

>>>
data
7

 

• Double Pre Underscore( __variable)

Name Mangling. Private access modifier. It called from within the class only. It prevents overriding variable in inherited class.

class Sample():
    def __init__(self):
        self.a=1
        self._b=2
        self.__c=3

class SecondClass(Sample):
    def __init__(self):
        super().__init__()
        self.a='a overridden'
        self._b='b overridden'
        self.__c='c overridden'
obj2=SecondClass()
print(obj2.a)
print(obj2._b)
print(obj2.__c)

>>>
a overridden
b overridden
AttributeError: 'SecondClass' object has no attribute '__c'

It occurs the error because __c could not find in the herited class.

Accessible from outside with "__class__variable".

print(obj2._SecondClass__c)

>>>
c overridden

 

Other example below.

_SimpleClass__name='datacamp'
class SimpleClass:
    def return_name(self):
        return __name
    
obj=SimpleClass()
print(obj.return_name())

>>>
datacamp

 

• Double Pre Underscore and Post Underscore(__variable__)

Magic methods. 

 

- __repr__

Represent the object.

class String:
    def __init__(self, string):
        self.string=string
        
    def __repr__(self) :
        return 'object:{}'.format(self.string)
        
if __name__=='__main__':
    string1=String('hello')
    print(string1)
    
>>>
object:hello

 

- __add__

Add the object.

class String:
    def __init__(self, string):
        self.string=string
        
    def __repr__(self) :
        return 'object:{}'.format(self.string)
    
    def __add__(self, other):
        return self.string+other
        
if __name__=='__main__':
    string1=String('hello')
    print(string1+'world')
    
>>>
helloworld

 

reference

- https://www.datacamp.com/tutorial/role-underscore-python

- https://eine.tistory.com/entry/%ED%8C%8C%EC%9D%B4%EC%8D%AC%EC%97%90%EC%84%9C-%EC%96%B8%EB%8D%94%EB%B0%94%EC%96%B8%EB%8D%94%EC%8A%A4%EC%BD%94%EC%96%B4-%EC%9D%98-%EC%9D%98%EB%AF%B8%EC%99%80-%EC%97%AD%ED%95%A0

- https://www.geeksforgeeks.org/dunder-magic-methods-python/