#!/usr/bin/env python
#-*- coding: utf-8-*-
# [SNIPPET_NAME: Simple metaclass]
# [SNIPPET_CATEGORIES: Python Core]
# [SNIPPET_DESCRIPTION: Shows how to create a and use a simple metaclass]
# [SNIPPET_AUTHOR: Florian Diesch <diesch@spamfence.net>]
# [SNIPPET_DOCS: http://www.ibm.com/developerworks/linux/library/l-pymeta.html]
# [SNIPPET_LICENSE: MIT]
# Copyright 2010 Florian Diesch <diesch@spamfence.net>
# All rights reserved.
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.
#
class PropertyMetaclass(type):
"""
This metaclass expects its instances to have a class attribute
__properties__ that is a dict mapping property names to their
default values. The metaclass creates the corresponding
properties
"""
def __init__(cls, name, bases, dict):
type.__init__(cls, name, bases, dict)
props = getattr(cls, '__properties__', {})
for name, default in props.iteritems():
type(cls).create_property(cls, name, default)
def attr_name(cls, prop):
""" returns the attribute name for property <prop>"""
return '_%s'%prop
def create_property(cls, name, default):
""" creates a property named <name> with default value <default>"""
getter=cls.create_getter(name)
setter=cls.create_setter(name)
prop=property(getter, setter, None, None)
# that's the attribute holding the actual value
setattr(cls, cls.attr_name(name), default)
# that's the property
setattr(cls, name, prop)
def create_getter(cls, prop):
""" creates a getter method for property <prop>"""
attr=cls.attr_name(prop)
def getter(self):
print " class %s: get %s"%(cls.__name__, prop)
return getattr(self, attr)
return getter
def create_setter(cls, prop):
""" creates a setter method for property <prop>"""
attr=cls.attr_name(prop)
def setter(self, value):
print " class %s: set %s to '%s'"%(cls.__name__, prop, value)
setattr(self, attr, value)
return setter
class Book(object):
__metaclass__= PropertyMetaclass # use the metaclass
__properties__ = { # some properties
'author': '[unknown title]',
'title': '[unknown author]'
}
if __name__ == '__main__':
book = Book()
print '%s by %s'%(book.title, book.author)
book.author = 'Euclid'
book.title = 'Elements'
print '%s by %s'%(book.title, book.author)
# prints:
#
# class Book: get title
# class Book: get author
# --unknown author-- by --unknown title--
# class Book: set author to 'Euclid'
# class Book: set title to 'Elements'
# class Book: get title
# class Book: get author
# Elements by Euclid
#//python/2371