How to add attribute to python class that is _not_ inherited?

Question

I need to be able to set a flag on a class (not on an instance of a class) which is not visible to a subclass. The question is, is it possible, and how would I do it if it is?

To illustrate, I want something like this:

class Master(SomeOtherClass):
    __flag__ = True

class Child(Master):
    pass

... where hasattr(Master, "__flag__") should return True for Master but False for Child. Is this possible? If so, how? I don't want to have to explicitly set __flag__ to false in every child.

My initial thought was to define __metaclass__, but I don't have the luxury of doing that because Master inherits from some other classes and metaclasses I don't control and which are private.

Ultimately I'm wanting to write a decorator so that I can do something like:

@hide_this
class Master(SomeOtherClass): pass

@hide_this
class Child(Master): pass

class GrandChild(Child): pass
...
for cls in (Master, Child, GrandChild)
    if cls.__hidden__:
        # Master, Child
    else:
        # GrandChild

You could make it __hidden, then only classes where it was directly defined would have a _ClassName__hidden attribute. — jonrsharpe
– jonrsharpe, Commented Jun 19, 2015 at 18:57
Is there a reason you don't want to write cls.__dict__.get('__hidden__'), other than that it's ugly? — Steve Jessop
– Steve Jessop, Commented Jun 19, 2015 at 18:59
@jonrsharpe: nope, that solution didn't occur to me. That will work fantastically. Write up an answer so I can accept it if you want. If you don't, I'll add the answer myself for future reference. Thanks! — Bryan Oakley
– Bryan Oakley, Commented Jun 19, 2015 at 19:47

jonrsharpe · Accepted Answer · 2015-06-19 19:58:26Z

6

You were very close:

class Master(SomeOtherClass):
    __flag = True

class Child(Master):
    pass

Two leading underscores without trailing underscores invokes name mangling, so the attribute will be named _Master__flag. Therefore if you check:

hasattr(cls, '_{}__flag'.format(cls.__name__))

it will only be True for Master, not Child.

edited Jun 19, 2015 at 19:58

answered Jun 19, 2015 at 19:52

jonrsharpe

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2 Comments

Bryan Oakley Over a year ago

That's not precisely what I'm looking for -- I don't have an instance of the class, I just have the class. The technique works the same, though.

jonrsharpe Over a year ago

@BryanOakley yes, it's the same with classes; updated accordingly!

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