How to allow only getter/setter to get/set variable value? [duplicate]

Question

In this example, a user can still access and set the variable using:

obj = C()
obj._x=10
varA  = obj._x

Even if we use __x instead of _x in below code, x still doesn't become private. So the only purpose of _x is to let the programmers know that it is not meant to be set directly via obj._x?

class C:
    def __init__(self):
        self._x = None

    @property
    def x(self):
        """I'm the 'x' property."""
        return self._x

    @x.setter
    def x(self, value):
        self._x = value

    @x.deleter
    def x(self):
        del self._x

Answer 1

In Python, it is a convention to use _var_name in order to indicate that something is private, but it does not enforce it like in languages as C++, C#, Java, etc.

A double underscore prefix causes the Python interpreter to rewrite the attribute name in order to avoid naming conflicts in subclasses.

This is also called name mangling—the interpreter changes the name of the variable in a way that makes it harder to create collisions when the class is extended later.

Consider the following example

class Test:
    def __init__(self):
        self.foo = 11
        self._bar = 23
        self.__baz = 23

Now, let's run the following code:

>>> t = Test()
>>> dir(t)
['_Test__baz', '__class__', '__delattr__', '__dict__', '__dir__',
 '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__',
 '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__module__',
 '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
 '__setattr__', '__sizeof__', '__str__', '__subclasshook__',
 '__weakref__', '_bar', 'foo']

dir(t) gives us a list with the object’s attributes. When you search for __baz in that list you’ll see that there is no variable with that name. If you look closely you’ll see there’s an attribute called _Test__baz on this object. This is the name mangling that the Python interpreter applies.