Class properties
Project description
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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'
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