Private Class Variables in Python?

Question

I'd like to be able to extend a class without inheriting one of the class variables.

Given this scenario:

class A:
    aliases=['a','ay']

class B(A):
    pass

print(B.aliases)

I would rather get an error that B has not defined the aliases variable rather than have B accidentally called ay.

One could imagine a solution where aliases becomes a member of the instantiated object (self.aliases) and is set in __init__ but I really want to be able to access the aliases using the cls object rather than an instance of the class.

Any suggestions?

see this answer: https://stackoverflow.com/questions/1641219/does-python-have-private-variables-in-classes — Dan, Sep 06 '19 at 14:31
Python just doesn't have the types of visibility restrictions that other languages do. (Also, what you do mean by "rather than have `B` accidentally called `ay`"?) — chepner, Sep 06 '19 at 14:40
Since "ay" is an alias to A but would also be used for B in the above. — Ray Salemi, Sep 06 '19 at 14:48
Nothing. I just wanted to force the error in case someone forgot. — Ray Salemi, Sep 06 '19 at 15:52

score 5 · Accepted Answer · answered Sep 06 '19 at 14:49

5

Python does not have REALY private attributes. But you can define it with a double underscore (__):

class A:
    __aliases=['a','ay']

class B(A):
    pass

print(B.__aliases) # yields AttributeError

But you still will be able to access it with:

print(B._A__aliases)

answered Sep 06 '19 at 14:49

Yevhen Kuzmovych

10,940
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Oh. This is just what I was hoping for. – Ray Salemi Sep 06 '19 at 14:53
And *that* is the solution! – Ray Salemi Sep 06 '19 at 14:54
I didn't realize the "__" mangling worked for class variables. – Ray Salemi Sep 06 '19 at 14:57

score 2 · Answer 2 · answered Sep 06 '19 at 14:46

2

This is kindof a ganky work around but here you go:

class K:
    def __init__(self):
        self.mems = dir(self)

def defaultMembers():
    k = K()
    return(k.mems)

class A:
    aliases=['a','ay']

class B(A):
    def __init__(self):
        for k in set(dir(self))-set(defaultMembers()):
            print("removing "+k)
            setattr(self, k, None)

a = A()
b = B()
print(b.aliases)
#None
print(a.aliases)
#['a','ay']

I guess all you really need is the setattr(self, "aliases", None) still this results in a None and not a non-variable. Unfortunately calsses don't support deletion because I tried to use del first.

answered Sep 06 '19 at 14:46

kpie

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5
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I think the point was to modify the behaviour when accessing `aliases` from the class object (B) not a class instance (b). – Dan Sep 06 '19 at 14:48
idk how he expects to work with class variables without instantiating them – SuperStew Sep 06 '19 at 14:49
I was hoping there was some simple workaround such as "__aliases" not getting copied on extension. – Ray Salemi Sep 06 '19 at 14:53
I'm thinking this answer should now be removed. Thanks! – Ray Salemi Sep 06 '19 at 15:02

Private Class Variables in Python?

2 Answers2