Friday, February 22, 2013

Encapsulation in Python

I was reading the discussion on Stackoverflow about encapsulation in Python

A user complained that he could call a private function by calling the mangling of internal names.

>>> class MyClass:
...     def myPublicMethod(self):
...             print 'public method'
...     def __myPrivateMethod(self):
...             print 'this is private!!'
>>> obj = MyClass()
>>> obj.myPublicMethod()
public method
>>> obj.__myPrivateMethod()
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: MyClass instance has no attribute '__myPrivateMethod'
>>> dir(obj)
['_MyClass__myPrivateMethod', '__doc__', '__module__', 'myPublicMethod']
>>> obj._MyClass__myPrivateMethod()
this is private!!
A good answer is, if this behavior is to be prevented, to check where the call was initiated

import re
import inspect

class MyClass :

    def __init__(self) :

    def private_function ( self ) :
        try :
            function_call = inspect.stack()[1][4][0].strip()

            # See if the function_call has "self." in the begining
            matched = re.match( '^self\.', function_call )
            if not matched :
                print 'This is Private Function, Go Away'
        except :
            print 'This is Private Function, Go Away'

        # This is the real Function, only accessible inside class #
        print 'Hey, Welcome in to function'

    def public_function ( self ) :
        # i can call private function from inside the class

### End ###

It's cool to use the internals of languages and compilers to access stuff that does not appear in the beginner's guide!

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