Private attributes and methods 2020

In the context of class, private means the attributes are only available for the members of the class not for the outside of the class.

Suppose we have the following class which has private attributes (__alias):

# p.py
class P:
   def __init__(self, name, alias):
      self.name = name       # public
      self.__alias = alias   # private

   def who(self):
      print('name  : ', self.name)
      print('alias : ', self.__alias)

We create an instance of class P, then trying to access its attributes whether it's public or private:

>>> from p import P
>>> x = P(name='Alex', alias='amen')
>>> x.name
'Alex'
>>> x.alias
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: P instance has no attribute 'alias'

For public attribute name, we can access through an instance variable, but not for the private attribute alias. Even we try this:

>>> x.__alias
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: P instance has no attribute '__alias'

still we're not allowed to access it.

But here is a magic want. One underscore('_') with the class name will do magic:

>>> x._P__alias
'amen'

The the following call also works as expected:

>>> x.who()
('name  : ', 'Alex')
('alias : ', 'amen')

We're going to use the same code as in the previous section but we'll add two methods: foo() and __foo() methods:

# p2.py
class P:
   def __init__(self, name, alias):
      self.name = name       # public
      self.__alias = alias   # private

   def who(self):
      print('name  : ', self.name)
      print('alias : ', self.__alias)

   def __foo(self):          # private method
      print('This is private method')

   def foo(self):            # public method
      print('This is public method')
      self.__foo()

How can we use the private method. If we try:

>>> from p2 import P
>>> x = P('Alex', 'amem')
>>> x.__foo()
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: P instance has no attribute '__foo'

The right way of accessing private method is this:

>>> x._P__foo()
This is private method

Of course, calling private method via public will work as expected:

>>> x.foo()
This is public emthod
This is private method