classproperty 1.0

Class properties

Author:

Philipp von Weitershausen

Email:

[e-mail]

License:

Zope Public License, v2.1

Motivation

Using method decorators and descriptors like property, we can easily create computed attributes:

>>> class JamesBrown(object):
...     @property
...     def feel(self):
...         return self._feel

An attribute like this cannot be written, though. You would have to do something like this:

>>> class JamesBrown(object):
...     def _getFeel(self):
...         return self._feel
...     def _setFeel(self, feel):
...         self._feel = feel
...     feel = property(_getFeel, _setFeel)

The problem with this approach is that it leaves the getter and setter sitting around in the class namespace. It also lacks the compact spelling of a decorator solution. To cope with that, some people like to write:

>>> class JamesBrown(object):
...     @apply
...     def feel():
...         def get(self):
...             return self._feel
...         def set(self, feel):
...             self._feel = feel
...         return property(get, set)

This spelling feels rather cumbersome, apart from the fact that apply is going to go away in Python 3000.

Goal

There should be a way to declare a read & write property and still use the compact and easy decorator spelling. The read & write properties should be as easy to use as the read-only property. We explicitly don’t want that immediately called function that really just helps us name the attribute and create a local scope for the getter and setter.

Class properties

Class properties let you define properties via the class statement. You define a dynamic property as if you were implementing a class. This works like this:

>>> from classproperty import classproperty
>>> class JamesBrown(object):
...     class feel(classproperty):
...         def __get__(self):
...             return self._feel
...         def __set__(self, feel):
...             self._feel = feel

>>> i = JamesBrown()
>>> i.feel
Traceback (most recent call last):
...
AttributeError: 'JamesBrown' object has no attribute '_feel'

>>> i.feel = "good"
>>> i.feel
'good'

Of course, deleters are also possible:

>>> class JamesBrown(object):
...     class feel(classproperty):
...         def __get__(self):
...             return self._feel
...         def __set__(self, feel):
...             self._feel = feel
...         def __delete__(self):
...             del self._feel

>>> i = JamesBrown()
>>> i.feel = "good"
>>> del i.feel
>>> i.feel
Traceback (most recent call last):
...
AttributeError: 'JamesBrown' object has no attribute '_feel'