How Properties and Descriptors Relate

As mentioned earlier, properties and descriptors are strongly related—the property built-in is just a convenient way to create a descriptor. Now that you know how both work, you should also be able to see that it’s possible to simulate the property built-in with a descriptor class like the following:

class Property:
    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        self.fget = fget
        self.fset = fset
        self.fdel = fdel                                  # Save unbound methods
        self.__doc__ = doc                                # or other callables

    def __get__(self, instance, instancetype=None):
        if instance is None:
            return self
        if self.fget is None:
            raise AttributeError("can't get attribute")
        return self.fget(instance)                        # Pass instance to self
                                                          # in property accessors
    def __set__(self, instance, value):
        if self.fset is None:
            raise AttributeError("can't set attribute")
        self.fset(instance, value)

    def __delete__(self, instance):
        if self.fdel is None:
            raise AttributeError("can't delete attribute")
        self.fdel(instance)

class Person:
    def getName(self): ...
    def setName(self, value): ...
    name = Property(getName, setName)                     # Use like property()

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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.

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.