Python private class variables that aren't class variables

Question

When trying to access __variables from a class, the parser assumes the 2 underscores are private relative to the current class. Notice how an unrelated function gets a "private" variable.

Is this a bug?

>>> def f(): pass
...
>>> class A:
...     def g(self):
...             f.__x = 1
...             def h():
...                     pass
...             h.__y = 2
...             return h
...
>>> z = A().g()
>>> dir(z)
['_A__y', '__call__', '__class__', '__delattr__', '__dict__', '__doc__', '__get_
_', '__getattribute__', '__hash__', '__init__', '__module__', '__name__', '__new
__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__', 'func_
closure', 'func_code', 'func_defaults', 'func_dict', 'func_doc', 'func_globals',
 'func_name']
>>> dir(f)
['_A__x', '__call__', '__class__', '__delattr__', '__dict__', '__doc__', '__get_
_', '__getattribute__', '__hash__', '__init__', '__module__', '__name__', '__new
__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__', 'func_
closure', 'func_code', 'func_defaults', 'func_dict', 'func_doc', 'func_globals',
 'func_name']

Tested on python 2.5 and 3.2

Working as expected. Granted, what you're doing with them is unusual, but those attributes belong to the class, wherever they happen to be. — kelm
– kelm, Commented Aug 6, 2011 at 13:30

senderle · Accepted Answer · 2011-08-06 13:28:08Z

2

This is a well documented behavior.

This mangling is done without regard to the syntactic position of the identifier, as long as it occurs within the definition of a class.

answered Aug 6, 2011 at 13:28

senderle

152k36 gold badges218 silver badges244 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

ubershmekel Over a year ago

Thing is, if you define f.__x from anywhere outside of a class definition it works fine. So "f" doesn't have any privacy (as some other already deleted answers called it), it's just that from within a class you can't access these kind of variables (ignoring getattr/setattr).

senderle Over a year ago

Sorry, access what kind of variables? Variables starting with __? I would say that it is a mistake to assign a value to f.__x outside of a class definition. A single underscore is sufficient to mark a variable as "private"; the purpose of name mangling is not really privacy but collision avoidance in the context of class inheritance.

ubershmekel Over a year ago

According to these facts, yes, I agree that it's a mistake to use double underscores outside of classes. It's just a surprising conclusion to me.

Sam-zhuang · Accepted Answer · 2021-08-24 15:01:47Z

0

Here’s a way to declare private variables: overload the __setattr__ (and other many methods about attributes) method, in the method, you can check current executing method is in the own class. If it is true, let it pass. Otherwise, you can raise an exception, or you can use your own customize error handle logic. (Tips: use inspect.stack function)

answered Aug 24, 2021 at 15:01

Sam-zhuang

245 bronze badges

Collectives™ on Stack Overflow

Python private class variables that aren't class variables

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related