Change the class attribute value outside the class

Question

I have a library like this:

class Robot:
 __counter = 0
 @classmethod
 def get_c(cls):
 result = cls.__counter + 1
 return result

I'm trying to access this class with initializing the class attribute __counter = 1, so I can get 2 from get_c() function. This is what I did:

Robot.__counter = 1
x = Robot()
x.get_c()

why the result is still 1 and what is the solution of doing what I want without touching the class internally thanks!

• juanpa.arrivillaga: Why oh why, are you using double-underscore name-mangling?

Answer 1

You've used double-underscore name-mangling. That translate every time you do:

__some_var

Inside a class definition into:

_MyClass__some_var

That's it's entire point - to avoid name-collisions in subclasses.

Just don't use double-underscore name-mangling if you want to access it outside the class like that.

I'm not sure what you mean precisely by "without touching the class internally", but in this case:

Robot._Robot__counter = 1

Would work, although doing the above is a sign you shouldn't be using double-underscore name-mangling.

As an aside, this is not equivalent to "private" in languages with access modifiers, although, it serves the same purpose for a limited use-case, preventing accidental name-collisions in subclasses.