Creating a New Kind of Class or Instance Attribute

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

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Problem

You want to create a new kind of instance attribute type with some extra functionality, such as type checking.

Solution

If you want to create an entirely new kind of instance attribute, define its functionality in the form of a descriptor class. Here is an example:

# Descriptor attribute for an integer type-checked attribute
class Integer(object):
    def __init__(self, name):
        self.name = name

    def __get__(self, instance, cls):
        if instance is None:
            return self
        else:
            return instance.__dict__[self.name]

    def __set__(self, instance, value):
        if not isinstance(value, int):
            raise TypeError('Expected an int')
        instance.__dict__[self.name] = value

    def __delete__(self, instance):
        del instance.__dict__[self.name]

A descriptor is a class that implements the three core attribute access operations (get, set, and delete) in the form of __get__(), __set__(), and __delete__() special methods. These methods work by receiving an instance as input. The underlying dictionary of the instance is then manipulated as appropriate.

To use a descriptor, instances of the descriptor are placed into a class definition as class variables. For example:

class Point(object):
    x = Integer('x')
    y = Integer('y')
    def __init__(self, x, y):
        self.x = x
        self.y = y

When you do this, all ...