Creating Managed Attributes

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

Visit the Python Cookbook: Classes and Objects Lab to test out this Shortcut in a live practice environment.

Problem

You want to add extra processing (e.g., type checking or validation) to the getting or setting of an instance attribute.

Solution

A simple way to customize access to an attribute is to define it as a “property.” For example, this code defines a property that adds simple type checking to an attribute:

class Person(object):
    def __init__(self, first_name):
        self.first_name = first_name

    # Getter function
    @property
    def first_name(self):
        return self._first_name

    # Setter function
    @first_name.setter
    def first_name(self, value):
        if not isinstance(value, str):
            raise TypeError('Expected a string')
        self._first_name = value

    # Deleter function (optional)
    @first_name.deleter
    def first_name(self):
        raise AttributeError("Can't delete attribute")

In the preceding code, there are three related methods, all of which must have the same name. The first method is a getter function and establishes first_name as a property. The other two methods attach optional setter and deleter functions to the first_name property. It’s important to stress that the @first_name.setter and @first_name.deleter decorators won’t be defined unless first_name was already established as a property using @property.

A critical feature of a property is that it looks like a normal attribute, but access automatically ...