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?

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
– chepner, Commented Sep 6, 2019 at 14:40
Since "ay" is an alias to A but would also be used for B in the above. — Ray Salemi
– Ray Salemi, Commented Sep 6, 2019 at 14:48

Yevhen Kuzmovych · Accepted Answer · 2019-09-06 14:49:48Z

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 6, 2019 at 14:49

Yevhen Kuzmovych

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3 Comments

Ray Salemi Over a year ago

Oh. This is just what I was hoping for.

Ray Salemi Over a year ago

And that is the solution!

Ray Salemi Over a year ago

I didn't realize the "__" mangling worked for class variables.

kpie · Accepted Answer · 2019-09-06 14:46:40Z

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 6, 2019 at 14:46

kpie

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4 Comments

Dan Over a year ago

I think the point was to modify the behaviour when accessing aliases from the class object (B) not a class instance (b).

SuperStew Over a year ago

idk how he expects to work with class variables without instantiating them

Ray Salemi Over a year ago

I was hoping there was some simple workaround such as "__aliases" not getting copied on extension.

Ray Salemi Over a year ago

I'm thinking this answer should now be removed. Thanks!

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