How to make attributes un-inheritable in Python using descriptors

April 6, 2013

For, and previous work at, we added the ability for the SymPy doctester to run or not run doctests conditionally depending on whether or not required external dependencies are installed. This means that for example we can doctest all the plotting examples without them failing when matplotlib is not installed.

For functions, this is as easy as decorating the function with @doctest_depends, which adds the attribute _doctest_depends_on to the function with a list of what dependencies the doctest depends on. The doctest will then not run the doctest unless those dependencies are installed.

For classes, this is not so easy. Ideally, one could just define _doctest_depends_on as an attribute of the class. However, the issue is that with classes, we have inheritance. But if class A has a docstring with a doctest that depends on some modules, it doesn’t mean that a subclass B of A will have a doctest that does.

Really, what we need to do is to decorate the docstring itself, not the class. Unfortunately, Python does not allow adding attributes to strings

>>> a = ""
>>> a.x = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'x'

So what we have to do is to create a attribute that doesn’t inherit.

I had for some time wanted to give descriptors in Python a try, since they are a cool feature, but also the second most complicated feature in Python (the first is metaclasses). If you don’t know what a descriptor is, I recommend reading this blog post by Guido van Rossum, the creator of Python. It’s the best explanation of the feature there is.

Basically, Python lets attributes define what happens when they are accessed (like a.x). You may already know that objects can define how their attributes are accessed via __getattr__. This is different. With descriptors, the attributes themselves define what happens. This may sound less useful, but in fact, it’s a very core feature of the language.

If you’ve ever wondered how property, classmethod, or staticmethod work in Python, the answer is descriptors. Basically, if you have something like

class A(object):
    def f(self):
        return 1
    f = property(f)

Then A().f magically calls what would normally be A().f(). The way it works is that property defines the __get__ method, which returns f(obj), where obj is the calling object, here A() (remember in Python that the first argument of a method, usually called self, is the object that calls the method).

Descriptors can allow a method to define arbitrary behavior when called, set, or deleted. To make an attribute inaccessible to subclasses, then, you just need to define a descriptor that prevents the attribute from being accessed if the class of the calling object is not the original class. Here is some code:

class nosubclasses(object):
    def __init__(self, f, cls):
        self.f = f
        self.cls = cls
    def __get__(self, obj, type=None):
        if type == self.cls:
            if hasattr(self.f, '__get__'):
                return self.f.__get__(obj, type)
            return self.f
        raise AttributeError

it works like this

In [2]: class MyClass(object):
   ...:     x = 1

In [3]: MyClass.x = nosubclasses(MyClass.x, MyClass)

In [4]: class MySubclass(MyClass):
   ...:     pass

In [5]: MyClass.x
Out[5]: 1

In [6]: MyClass().x
Out[6]: 1

In [80]: MySubclass.x
AttributeError                            Traceback (most recent call last)
<ipython-input-80-2b2f456dd101> in <module>()
----> 1 MySubclass.x

<ipython-input-51-7fe1b5063367> in __get__(self, obj, type)
      8                 return self.f.__get__(obj, type)
      9             return self.f
---> 10         raise AttributeError


In [81]: MySubclass().x
AttributeError                            Traceback (most recent call last)
<ipython-input-81-93764eeb9948> in <module>()
----> 1 MySubclass().x

<ipython-input-51-7fe1b5063367> in __get__(self, obj, type)
      8                 return self.f.__get__(obj, type)
      9             return self.f
---> 10         raise AttributeError


Note that by using the third argument to __get__, this works regardless if the attribute is accessed from the class or the object. I have to call __get__ on self.f again if it has it to ensure that the right thing happens if the attribute has other descriptor logic defined (and note that regular methods have descriptor logic defined—that’s how they convert the first argument self to implicitly be the calling object).

One could easily make class decorator that automatically adds the attribute to the class in a non-inheritable way:

def nosubclass_x(args):
    def _wrapper(cls):
        cls.x = nosubclasses(args, cls)
        return cls
    return _wrapper

This automatically adds the property x to the decorated class with the value given in the decorator, and it won’t be accessible to subclasses:

In [87]: @nosubclass_x(1)
   ....: class MyClass(object):
   ....:     pass

In [88]: MyClass().x
Out[88]: 1

In [89]: MySubclass().x
AttributeError                            Traceback (most recent call last)
<ipython-input-89-93764eeb9948> in <module>()
----> 1 MySubclass().x

<ipython-input-51-7fe1b5063367> in __get__(self, obj, type)
      8                 return self.f.__get__(obj, type)
      9             return self.f
---> 10         raise AttributeError


For SymPy, we can’t use class decorators because we still support Python 2.5, and they were introduced in Python 2.6. The best work around is to just call Class.attribute = nosubclasses(Class.attribute, Class) after the class definition. Unfortunately, you can’t access a class inside its definition like you can with functions, so this has to go at the end.

Name Mangling

After coming up with all this, I remembered that Python already has a pretty standard way to define attributes in such a way that subclasses won’t have access to them. All you have to do is use two underscores before the name, like __x, and it will be name mangled. This means that the name will be renamed to _classname__x outside the class definition. The name will not be inherited by subclasses. There are some subtleties with this, particularly for strange class names (names that are too long, or names that begin with an underscore). I asked about this on StackOverflow. The best answer is that there was a function in the standard library, but it was removed in Python 3. My tests reveal that the behavior is different in CPYthon than in PyPy, so getting it right for every possible class is nontrivial. The descriptor thing should work everywhere, though. On the other hand, getattr(obj, '_' + obj.__class__.__name__ + attributename) will work 99% of the time, and is much easier both to write and to understand than the descriptor.

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