How to resolve infinite recursion in getattribute with class variables

Question

I have a base class which is inherited from by multiple classes. In the child classes, I want to make a method from the base class uncallable; so I'm using __getattribute__ to raise an error if they are tried from the child class. Dummy example below:

class Base(object):
    def f1():
        return 1

    def f2():
        return 2

    def f3():
        return 3


class Child1(Base):
     def __getattribute__(self, name):
        if name == 'f1':
            raise AttributeError(name)
        else:
            return super(Child1, self).__getattribute__(name)


class Child2(Base):
     def __getattribute__(self, name):
        if name == 'f2':
            raise AttributeError(name)
        else:
            return super(Child2, self).__getattribute__(name)


class Child3(Base):
     def __getattribute__(self, name):
        if name == 'f3':
            raise AttributeError(name)
        else:
            return super(Child3, self).__getattribute__(name)

So in this example, I don't want to be able to call f1 from Child1, f2 from Child2, and f3 from Child3.

Is there a way to re-write this that doesn't have so much code duplication and child classes just inherit the method? This is what I tried, but it first runs into recursion issues because the call to self.blocked_methods in and of itself calls __getattribute__. I imagine there are other issues but can't get past that one.

class Base(object):
    blocked_methods = []

    def f1():
        return 1

    def f2():
        return 2

    def f3():
        return 3

    def __getattribute__(self, name):
        if name in self.blocked_methods:
            raise AttributeError(name)
        else:
            return super(Child3, self).__getattribute__(name)


class Child1(Base):
     blocked_methods = ['f1']


class Child2(Base):
     blocked_methods = ['f2']


class Child3(Base):
     blocked_methods = ['f3']

"In certain classes, I want to make a method from the base class uncallable" You understand that this is explicitly the opposite of what inheritance is supposed to be for, yes? — Karl Knechtel
– Karl Knechtel, Commented Sep 10, 2020 at 20:08
This suggests that Base is not the appropriate base class for any of the three child classes. Consider defining separate mixin classes for each of the three methods instead. — chepner
– chepner, Commented Sep 10, 2020 at 20:09

chepner · Accepted Answer · 2020-09-10 20:11:14Z

1

Use multiple mix-in classes instead.

class Method1:
    def f1(self):
        return 1

class Method2:
    def f2(self):
        return 2

class Method3:
    def f3(self):
        return 3

class Child1(Method2, Method3):
    pass

class Child2(Method1, Method3):
    pass

class Child1(Method2, Method2):
    pass

It's not terribly scalable, but it's better than trying to subtract functionality from a class.

answered Sep 10, 2020 at 20:11

chepner

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1 Comment

TomNash Over a year ago

This looks like the best approach for what I'm after, there would only be 3-4 mix-in classes. This actually works perfectly as now I can isolate and group functions specific to that mix-in.

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