Truly Private Variables in Python 3

Question

So I know the way to make a variable "private" in python like this:

class Foo:
    def __init__(self):
        self.__private = 'bar'

This "works" and doesn't, as shown below:

foo = Foo()
'__private' in vars(foo) #False
'_Foo__private' in vars(foo) #True

Now, I understand this is the way to make private variables in python and I like this way. It allows you to mangle names so that no subclasses accidentally override this (because it begins with the class's name), and that nobody will accidentally use it. It also gives you the power to change the private variables if you know what you are doing. Also, it is the best way to do it, because truly private variables are impossible.

Or so I thought.

Recently, I was reading PEP 8 and I saw this line:

We don't use the term "private" here, since no attribute is really private in Python (without a generally unnecessary amount of work).

This quote is found in the Designing for Inheritance section of PEP 8.

Note the phrase "without a generally unnecessary amount of work". I am now sure that there must be a way to get truly private variables in python. How would I do that?

I have tried overriding __getattribute__, but the problem is that there is no way to tell if the call is coming from inside the class or not (that I am aware of).

Also, the __dict__ attribute is annoying when trying to do this because it holds references to all instance variables.

I also thought of metaclasses, but those seem to have the same problems as __getattribute__.

Thoughts?

---

Note: I understand that any way to make truly private variables in python should never be done in productive code. I just want to know how it could be done.

Answer 1

You can get nearly the same effect without the fancy inspection by using closures instead of attributes.

class Foo:
    def __init__(self):
        private = 'bar'
        def print_private():
            print(private)
        self.print_private = print_private

foo = Foo()
foo.print_private()  # works
foo.private  # kaboom

Of course, inspect can see into closures too.