Why are Python's 'private' methods not

Question

Python gives us the ability to create 'private' methods and variables within a class by prepending double underscores to the name, like this: __myPrivateMethod(). How, then, can one explain this

>>> 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!!

What's the deal?!

I'll explain this a little for those who didn't quite get that.

>>> class MyClass:
...     def myPublicMethod(self):
...             print 'public method'
...     def __myPrivateMethod(self):
...             print 'this is private!!'
... 
>>> obj = MyClass()

What I did there is create a class with a public method and a private method and instantiate it.

Next, I call its public method.

>>> obj.myPublicMethod()
public method

Next, I try and call its private method.

>>> obj.__myPrivateMethod()
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: MyClass instance has no attribute '__myPrivateMethod'

Everything looks good here; we're unable to call it. It is, in fact, 'private'. Well, actually it isn't. Running dir() on the object reveals a new magical method that python creates magically for all of your 'private' methods.

>>> dir(obj)
['_MyClass__myPrivateMethod', '__doc__', '__module__', 'myPublicMethod']

This new method's name is always an underscore, followed by the class name, followed by the method name.

>>> obj._MyClass__myPrivateMethod()
this is private!!

So much for encapsulation, eh?

In any case, I'd always heard Python doesn't support encapsulation, so why even try? What gives?

Answer 1

When I first came from Java to Python I hated this. It scared me to death.

Today it might just be the one thing I love most about Python.

I love being on a platform, where people trust each other and don't feel like they need to build impenetrable walls around their code. In strongly encapsulated languages, if an API has a bug, and you have figured out what goes wrong, you may still be unable to work around it because the needed method is private. In Python the attitude is: "sure". If you think you understand the situation, perhaps you have even read it, then all we can say is "good luck!".

Remember, encapsulation is not even weakly related to "security", or keeping the kids off the lawn. It is just another pattern that should be used to make a code base easier to understand.

Answer 2

The most important concern about private methods and attributes is to tell developers not to call it outside the class and this is encapsulation. one may misunderstand security from encapsulation. when one deliberately uses syntax like that(bellow) you mentioned, you do not want encapsulation.

obj._MyClass__myPrivateMethod()

I have migrated from C# and at first it was weird for me too but after a while I came to the idea that only the way that Python code designers think about OOP is different.

Answer 3

It's just one of those language design choices. On some level they are justified. They make it so you need to go pretty far out of your way to try and call the method, and if you really need it that badly, you must have a pretty good reason!

Debugging hooks and testing come to mind as possible applications, used responsibly of course.

Answer 4

Similar behavior exists when module attribute names begin with a single underscore (e.g. _foo).

Module attributes named as such will not be copied into an importing module when using the from* method, e.g.:

from bar import *

However, this is a convention and not a language constraint. These are not private attributes; they can be referenced and manipulated by any importer. Some argue that because of this, Python can not implement true encapsulation.

Answer 5

The name scrambling is used to ensure that subclasses don't accidentally override the private methods and attributes of their superclasses. It's not designed to prevent deliberate access from outside.

For example:

>>> class Foo(object):
...     def __init__(self):
...         self.__baz = 42
...     def foo(self):
...         print self.__baz
...     
>>> class Bar(Foo):
...     def __init__(self):
...         super(Bar, self).__init__()
...         self.__baz = 21
...     def bar(self):
...         print self.__baz
...
>>> x = Bar()
>>> x.foo()
42
>>> x.bar()
21
>>> print x.__dict__
{'_Bar__baz': 21, '_Foo__baz': 42}

Of course, it breaks down if two different classes have the same name.

Answer 6

With Python 3.4 this is the behaviour:

>>> class Foo:
        def __init__(self):
                pass
        def __privateMethod(self):
                return 3
        def invoke(self):
                return self.__privateMethod()


>>> help(Foo)
Help on class Foo in module __main__:

class Foo(builtins.object)
 |  Methods defined here:
 |
 |  __init__(self)
 |
 |  invoke(self)
 |
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |
 |  __dict__
 |      dictionary for instance variables (if defined)
 |
 |  __weakref__
 |      list of weak references to the object (if defined)

 >>> f = Foo()
 >>> f.invoke()
 3
 >>> f.__privateMethod()
 Traceback (most recent call last):
   File "<pyshell#47>", line 1, in <module>
     f.__privateMethod()
 AttributeError: 'Foo' object has no attribute '__privateMethod'

https://docs.python.org/3/tutorial/classes.html#tut-private

Note that the mangling rules are designed mostly to avoid accidents; it still is possible to access or modify a variable that is considered private. This can even be useful in special circumstances, such as in the debugger.

Even if the question is old I hope my snippet could be helpful.