Python Cookbook

Implementing Properties

Credit: Luther Blissett

Problem

You want client code to use normal attribute-access syntax for using, binding, or deleting instance attributes, but you want the semantics of these actions to be determined by method calls (e.g., to compute an attribute’s value on the fly).

Solution

With Python 2.2 new-style classes, the new built-in property function lets you do this directly:

class Rectangle(object):
    def _ _init_ _(self, width, height):
        self.width = width
        self.height = height
    def getArea(self): return self.width * self.height
    def setArea(self, value): raise AttributeError, "Can't set 'area' attribute"
    area = property(getArea, setArea)

With classic classes, you must implement properties yourself with the special methods _ _getattr_ _ and _ _setattr_ _:

class Rectangle:
    def _ _init_ _(self, width, height):
        self.width = width
        self.height = height
    def getArea(self): return self.width * self.height
    def setArea(self, value): raise AttributeError, "Can't set 'area' attribute"
    def _ _getattr_ _(self, name):
        if name=='area': return self.getArea(  )
        raise AttributeError, name
    def _ _setattr_ _(self, name, value):
        if name=='area': return self.setArea(value)
        self._ _dict_ _[name] = value

Discussion

Properties are an important object-oriented concept. Instances of a class often need to expose two different kinds of attributes: those that hold data and those that are computed on the fly with a suitable method, whenever their values are required. If you expose the real attributes ...