when to make attributes private?

[snippsat]

I almost never use the double underscore privacy mechanism such as __eggs

The wording privacy mechanism is maybe not best description for the double underscore,as it a has "little" or nothing to do with privacy.
It's really to avoid naming conflicts in subclasses,also called name mangling.

To take quick tour of _ and __.
private and public variables like Java does,dos not translate directly over to Python.

---

Single _

class Bar:
    def __init__(self):
        self.external = 99
        self._internal = 50

Use:

>>> obj = Bar()
>>> 
>>> # get
>>> obj._internal
50
>>> # set
>>> obj._internal = 444
>>> obj._internal
444

Have full access to the attribute value of that variable even it with _.
_ just as Waring hint merely an agreed upon convention.

Hey, this isn’t really meant to be a part of the public interface of this class.
Best to leave it alone.

Nothing is really private in python.
After all, we're all consenting adults here.

---

Double __

class Foo:
    def __init__(self):
        self.var_1 = 'hello'
        self.__var_2 = 'egg' # Not private,it's for name mangling when subclass

class Bar(Foo):
    def __init__(self):
        super().__init__()
        self.var_1 = 'world'
        self.__var_2 = 'spam'

Use:

>>> obj = Bar()
>>> 
>>> obj.var_1
'world'
>>> # var_1 "hello" is overridden and can never be restored
>>> 
>>> obj._Foo__var_2
'egg'
>>> obj._Bar__var_2
'spam'
>>> # var_2 can not be overridden in subclass,because of name mangling so both "egg" and "spam" can be reached

So i guess we could say that we private copy of original egg,but it still is possible to access or modify a variable that we call private copy