python怎么创建和使用一个简单的元类

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示例

#!/usr/bin/env python# [SNIPPET_NAME: Simple metaclass]# [SNIPPET_CATEGORIES: Python Core] # [SNIPPET_DESCRIPTION: Shows how to create a and use a simple metaclass]# [SNIPPET_AUTHOR: Florian Diesch ]# [SNIPPET_LICENSE: MIT]# Copyright 2010 Florian Diesch # 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 """        return '_%s'%prop    def create_property(cls, name, default):        """ creates a property named  with default value """        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 """        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 """        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 setterclass 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

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