Can access class private variable?

[Michael]

Hi,

Am new to Python3 (previously Java)
Below I make a class, instantiate it, and then do 2 things I don't understand (see comments)

Appreciate some help - Michael

class HWClass:

    def __init__(self): 
        self.__classvar = 3 #private
        self.printHW()

    def printHW(self): 
        print("Hello World")

hw = HWClass()  #Instantiation
hw.__classvar = 4  #This suggests I change a private class variable? It doesn't actually change, but the text suggests it does, which is error-prone
hw.varxxxx = 9 #How can I use a variable on this class object that is not defined in the class?

[ichabod801]

Actually, hw.__classvar does change. Python does not have private variables. There is name mangling, but that's really not worth messing with. There's also properties, but that's more about setters/getters than private variables.

Also, classes accept new variables in their namespace. That's just how Python works. You can limit this behavior with __slots__, but that is rarely, if ever, done.

Python is more about making it easy to do things than it is about making rules that you can't do things.

[snippsat]

You can access it bye hw._HWClass__classvar  not hw.__classvar.

>>> hw = HWClass()
>>> hw._HWClass__classvar
3
>>> hw._HWClass__classvar = 4
>>> hw._HWClass__classvar
4

As mention there is nothing private in Python.
__ would say it should not be used before understanding what's it's used for(name mangling).
A quick demo of name mangling.

class HWClass:
    def __init__(self):
        self.__classvar = 3 # Not private, it's for name mangling

class HW(HWClass):
    def __init__(self):
        super().__init__()
        self.__classvar = 3 

Now if use Class HW,
has class variable classvar been named mangled(not overwritten).

>>> hw = HW()
# Both have been given different name(name mangling)
>>> hw._HWClass__classvar
3
>>> hw._HW__classvar
3

# Change one will not change the other
>>> hw._HWClass__classvar = 100
>>> hw._HWClass__classvar
100
>>> hw._HW__classvar
3

It's take some chaining to learn it the Python way,when coming from Java.