Learning Python - Mark Lutz [513]
This Property class catches attribute accesses with the descriptor protocol and routes requests to functions or methods passed in and saved in descriptor state when the class is created. Attribute fetches, for example, are routed from the Person class, to the Property class’s __get__ method, and back to the Person class’s getName. With descriptors, this “just works.”
Note that this descriptor class equivalent only handles basic property usage, though; to use @ decorator syntax to also specify set and delete operations, our Property class would also have to be extended with setter and deleter methods, which would save the decorated accessor function and return the property object (self should suffice). Since the property built-in already does this, we’ll omit a formal coding of this extension here.
Also note that descriptors are used to implement Python’s __slots__; instance attribute dictionaries are avoided by intercepting slot names with descriptors stored at the class level. See Chapter 31 for more on slots.
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Note
In Chapter 38, we’ll also make use of descriptors to implement function decorators that apply to both functions and methods. As you’ll see there, because descriptors receive both descriptor and subject class instances they work well in this role, though nested functions are usually a simpler solution.
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__getattr__ and __getattribute__
So far, we’ve studied properties and descriptors—tools for managing specific attributes. The __getattr__ and __getattribute__ operator overloading methods provide still other ways to intercept attribute fetches for class instances. Like properties and descriptors, they allow us to insert code to be run automatically when attributes are accessed; as we’ll see, though, these two methods can be used in more general ways.
Attribute fetch interception comes in two flavors, coded with two different methods:
__getattr__ is run for undefined attributes—that is, attributes not stored on an instance or inherited from one of its classes.
__getattribute__ is run for every attribute, so when using it you must be cautious to avoid recursive loops by passing attribute accesses to a superclass.
We met the former of these in Chapter 29; it’s available for all Python versions. The latter of these is available for new-style classes in 2.6, and for all (implicitly new-style) classes in 3.0. These two methods are representatives of a set of attribute interception methods that also includes __setattr__ and __delattr__. Because these methods have similar roles, we will generally treat them as a single topic here.
Unlike properties and descriptors, these methods are part of Python’s operator overloading protocol—specially named methods of a class, inherited by subclasses, and run automatically when instances are used in the implied built-in operation. Like all methods of a class, they each receive a first self argument when called, giving access to any required instance state information or other methods of the class.
The __getattr__ and __getattribute__ methods are also more generic than properties and descriptors—they can be used to intercept access to any (or even all) instance attribute fetches, not just the specific name to which they are assigned. Because of this, these two methods are well suited to general delegation-based coding patterns—they can be used to implement wrapper objects that manage all attribute accesses for an embedded object. By contrast, we must define one property or descriptor for every attribute we wish to intercept.
Finally, these two methods are more narrowly focused than the alternatives we considered earlier: they intercept attribute fetches only, not assignments. To also catch attribute changes by assignment, we must code a __setattr__ method—an operator overloading method run for every attribute fetch, which must take care to avoid recursive loops by routing attribute assignments through the instance namespace dictionary.
Although much less common, we can also code a __delattr__ overloading method (which must