Descriptors

Descriptors provide an alternative way to intercept attribute access; they are strongly related to the properties discussed in the prior section. In fact, a property is a kind of descriptor—technically speaking, the property built-in is just a simplified way to create a specific type of descriptor that runs method functions on attribute accesses.

Functionally speaking, the descriptor protocol allows us to route a specific attribute’s get and set operations to methods of a separate class object that we provide: they provide a way to insert code to be run automatically on attribute access, and they allow us to intercept attribute deletions and provide documentation for the attributes if desired.

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Descriptors are created as independent classes, and they are assigned to class attributes just like method functions. Like any other class attribute, they are inherited by subclasses and instances. Their access-interception methods are provided with both a self for the descriptor itself, and the instance of the client class. Because of this, they can retain and use state information of their own, as well as state information of the subject instance. For example, a descriptor may call methods available in the client class, as well as descriptor-specific methods it defines.

Like a property, a descriptor manages a single, specific attribute; although it can’t catch all attribute accesses generically, it provides control over both fetch and assignment accesses and allows us to change an attribute freely from simple data to a computation without breaking existing code. Properties really are just a convenient way to create a specific kind of descriptor, and as we shall see, they can be coded as descriptors directly.

Whereas properties are fairly narrow in scope, descriptors provide a more general solution. For instance, because they are coded as normal classes, descriptors have their own state, may participate in descriptor inheritance hierarchies, can use composition to aggregate objects, and provide a natural structure for coding internal methods and attribute documentation strings.