Can python private variables even be changed without using object._className__attributeName?

Question

consider the following code:

class animal:
    def __init__(self, name):
        self.__name = name

class dog(animal):
    def __init__(self, owner, name):
        self.__owner = owner
        super(dog, self).__init__(name)

terrier = dog('A', 'B')
terrier.owner = 'C'
terrier.name = 'D'
print(terrier._dog__owner, terrier._animal__name)
print(terrier.owner, terrier.name)

The output is:

A B
C D

I understand that Python private variables are only protected by convention. But all other threads mention ._className__attributeName eg. terrier._dog__owner as the only way of altering variable values. Here I am able to alter them even using terrier.owner or terrier.name.

Strangely, both give a different output as shown above. So do terrier.owner or terrier.name create different instances from terrier._dog__name or terrier._animal__name? What exactly has happened here?

Yes, terrier.owner is a different name from terrier._dog__owner. Why would you think otherwise? It's no different from doing terrier.blah = "foo". You can create whatever attributes you want. — BrenBarn
– BrenBarn, Commented Mar 25, 2016 at 18:34
Thanks but can you explain why the two separate instances of object terrier are being created that give two different sets of outputs? — Ananya Chandra
– Ananya Chandra, Commented Mar 25, 2016 at 18:35
@AnanyaChandra: all you did was add additional attributes. Those are independent from the double-underscore attributes. — Martijn Pieters
– Martijn Pieters, Commented Mar 25, 2016 at 18:36

Martijn Pieters · Accepted Answer · 2016-03-25 18:51:58Z

1

Double-underscore variables are indeed not private the way Java or C++ see privacy. They are not meant to be seen as private to code outside the class. They are used to avoid name clashes among subclasses, which is why the classname is used as a prefix.

E.g. when you have an internal implementation for a class Foo, and you don't want the subclass Bar(Foo) to accidentally use the same names in their implementation, you can use the attribute __spam, which will be transformed to _Foo__spam and _Bar__spam respectively, and thus won't clash.

The terrier.owner and terrier.name attributes you added are independent and have nothing to do with the _dog__owner and _animal__name attributes used by your classes. Using double-underscore variable names does not prevent other attributes from being created, including the same name without underscores.

edited Mar 25, 2016 at 18:51

answered Mar 25, 2016 at 18:35

Martijn Pieters

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1 Comment

Ananya Chandra Over a year ago

Coming from Java I thought terrier.__owner is a private version of terrier.owner. They are independent though with no relation whatsoever. Got it.

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