Descriptors generally have to interact with attributes of the managed object, and this is done by inspecting __dict__ on that object (or calling getattr/setattr, but the problem is the same), and finding the key under the specific name.

For this reason, the descriptor will have to know the name of the key to look for, which is related to the name of the attribute is managing.

On previous versions of Python this had to be done explicitly. If we wanted to work around it, there were some more advanced ways to do so. Luckily, after PEP-487 (added in Python 3.6), there are some enhancements regarding class creation, which also affects descriptors.

Let's review the problem, the previous approaches to tackle it, and the modern way of solving it.

Configure the name of the descriptor

The descriptor needs to somehow know which attribute will be modifying, and for this, the most common solution is to store the attribute name internally. For example in:

class LoggedAttr:
    def __init__(self, name=None): = name

    def __get__(self, instance, owner):
        if instance is None:
            return self
        return instance.__dict__[]

class Managed:
    descriptor = LoggedAttr('descriptor')

What we require is to check that the name is passed to the descriptor properly, basically:

    assert == 'descriptor'

But we don't want to pass the string 'descriptor' as a parameter when constructing it, because it's repetitive. Instead, we want this to be configured automatically. Let's see some options.

A class decorator

With a class decorator, we could define all decorators for the class as parameters of the decorator, and make the assignment of the name in it as well.

Something like this:

class configure_descriptors:
    def __init__(self, **kwargs):
        self.descs = {dname: dcls(dname) for dname, dcls in kwargs.items()}

    def __call__(self, class_):
        for dname, descriptor in self.descs.items():
            setattr(class_, dname, descriptor)
        return class_

class DecoratedManaged:
    """The descriptor is provided by the decorator"""

The condition is preserved:

assert == 'descriptor'

In this decorator, we provide the name and the class of the descriptor to be created, and the decorator instantiates the class with this name. We could also have created the instance directly in the descriptor, and then update the value with setattr(descriptor, 'name', dname), which is more general, in case you want to create descriptors that take multiple arguments on their __init__ method, but for this case it's just fine.

Then we set the new descriptor (the one that has the name already updated on it), to the wrapped class.

However, it still seems a bit unfamiliar or counter-intuitive that we're defining the descriptor not in the body of the class, but as a parameter of a decorator.

There must be another way.

A meta-class

Imagine we flag the class by adding a __set_name = True attribute on it, in order to hint the meta-class that this is going to be one of the attributes that need its name changed. Then the meta-class would look something like:

class MetaDescriptor(type):
    def __new__(cls, clsname, bases, cls_kwargs):
        for attrname, cls_attr in cls_kwargs.items():
            mangled_attr = "_{0}__set_name".format(cls_attr.__class__.__name__)
            if hasattr(cls_attr, mangled_attr):
                setattr(cls_attr, 'name', attrname)
        return super().__new__(cls, clsname, bases, cls_kwargs)

class MetaManaged(metaclass=MetaDescriptor):
    descriptor = LoggedAttr()

And again:

assert == 'descriptor'

One detail is that the __init__ of the descriptor accepts the name to be nullable so this works. Another option would have been defining only the descriptor assigned to the class, and then, re-mapping the attribute with the instance, passing the name when it's being constructed on the meta-class. Both options are the same, and the example was made with simplicity in mind.

This works but it has a couple of issues. First we have to somehow identify when the class attribute needs to be updated (in this case, a flag was added to it, but other alternatives are no better at all). The second problem should be rather obvious: it's not a good use of meta-classes, and this is overkill (to say the least) for what should be a simple task.

There must be a better way.

^^^^^^^^^^^^^ And there is. At least for Python 3.6 and higher. The __set_name__ method was included, which is automatically called when the class is being created, and it receives two parameters: the class and the name of the attribute as it appears defined in the class.

With this, the problem is reduced to just simply:

class LoggedAttr:
    def __set_name__(self, owner, name): = name

And that's it, no other code is needed. The solution is much simpler, and it entails less problems.

Actually, I deliberately named the flag __set_name, to get an idea of what's coming, and to hint that with __set_name__, Python must be doing something similar to the example, but in this case we shouldn't worry about it.


Even though it's fine to just know about the last method, and we could simply use that, it's still important to have followed this path, thinking about how things were done previously, because it's not fair to just assume things were always good, and take that for granted. Otherwise, we would miss the evolution of the language, and assume there were never issues, problems or things that needed revision.

And more importantly, there still are. Python still has lots of other areas for improvement. Just as in this example __set_name__ seems to solve a small, yet annoying problem, there are many other scenarios on which things are not crystal clear in Python, so the language still needs to evolve.