What is the meaning of single underscore prefix with Python variables?

Python variable name may begin with a single underscore. It functions as a convention to indicate that the variable name is now a private variable. It should be viewed as an implementation detail that could change at any time. Programmers can assume that variables marked with a single underscore are reserved for internal usage.

Single underscores are advised for semi-private variables, and double underscores are advised for fully private variables.

To paraphrase PEP-8; single leading underscore: a poor signal of "internal use." For instance, from M import * excludes objects with names that begin with an underscore.

Syntax

The syntax used for the single pre underscore with Python variables is as follows −

_name

Example 1

The buzz variable is made private in the example below by adding a single underscore before it. This approach still permits access to the _buzz variable when we instantiate an object of this class and try to access the _buzz variable. It is referred to as a "weak private indication" for this reason.

class Python:
   def __init__(self):
      self.fee = 37
      self._buzz = 76

object = Python()
print(object.fee)
print(object._buzz)

Output

Following is an output of the above code −

37 
76

Example 2

Following is an example of single leading underscore (_v):

class Sports:
   def __init__(self):
      self.name = 'Cricket'
      self._player = 'Sachin Tendulkar'

Output

Following is an output of the above code where we tried to access the ‘name’ and ‘player −

>>> s = Sports()
>>> s.name
'Cricket'
>>> s._player
'Sachin Tendulkar'

Note − Accessing the single pre underscore variable is not prevented by single pre underscore.However, a single pre underscore has an impact on the names imported from the module.

Example 3

Written the following code in untitled.py file

# The file name is ‘untitled.py’
def function():
   return "programming"

def _private_function():
return 35

Output

Python doesn't import names that begin with a single pre-underscore if you import all the methods and names from untitled.py.

Following is an output of the above code −

>>> from untitled import *
>>> function()
'programming'
>>> _private_function()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name '_private_function' is not defined

The modules should be imported normally to avoid the above error −

>>> import untitled
>>> untitled.function()
'programming'
>>> untitled._private_function()
35

Note − Single Pre Underscore is solely meant for internal use.

Example-4

The difference between double and single underscore prefixes is demonstrated in the following code.

class Python():
   def __init__(self):
      self.__FullPrivate = "Coding"
      self._SemiPrivate = "Programs"

p = Python()
print ('p._SemiPrivate')
print ('p.__FullPrivate')

Output

Following is an output of the above code −

p._SemiPrivate
p.__FullPrivate