What is a DynamicClassAttribute and how do I use it?

Question

As of Python 3.4, there is a descriptor called DynamicClassAttribute. The documentation states:

types.DynamicClassAttribute(fget=None, fset=None, fdel

Answer 1

What is a DynamicClassAttribute

A DynamicClassAttribute is a descriptor that is similar to property. Dynamic is part of the name because you get different results based on whether you access it via the class or via the instance:

• instance access is identical to property and simply runs whatever method was decorated, returning its result

• class access raises an AttributeError; when this happens Python then searches every parent class (via the mro) looking for that attribute -- when it doesn't find it, it calls the class' metaclass's __getattr__ for one last shot at finding the attribute. __getattr__ can, of course, do whatever it wants -- in the case of EnumMeta __getattr__ looks in the class' _member_map_ to see if the requested attribute is there, and returns it if it is. As a side note: all that searching had a severe performance impact, which is why we ended up putting all members that did not have name conflicts with DynamicClassAttributes in the Enum class' __dict__ after all.

and how do I use it?

You use it just like you would property -- the only difference is that you use it when creating a base class for other Enums. As an example, the Enum from aenum¹ has three reserved names:

• name
• value
• values

values is there to support Enum members with multiple values. That class is effectively:

class Enum(metaclass=EnumMeta):

    @DynamicClassAttribute
    def name(self):
        return self._name_

    @DynamicClassAttribute
    def value(self):
        return self._value_

    @DynamicClassAttribute
    def values(self):
        return self._values_

and now any aenum.Enum can have a values member without messing up Enum..values.

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¹ Disclosure: I am the author of the Python stdlib Enum, the enum34 backport, and the Advanced Enumeration (aenum) library.