原型模式

一、内容

用原型实例指定创建对象的种类，并且通过拷贝这些原型创建新的对象。
原型模式本质就是克隆对象，所以在对象初始化操作比较复杂的情况下，很实用，能大大降低耗时，提高性能，因为“不用重新初始化对象，而是动态地获得对象运行时的状态”。

浅拷贝（Shallow Copy）:指对象的字段被拷贝，而字段引用的对象不会被拷贝，拷贝的对象和源对象只是名称相同，但是他们共用一个实体。
深拷贝（deep copy）:对对象实例中字段引用的对象也进行拷贝。

二、代码实例

import copy
from collections import OrderedDict


class Book:
    def __init__(self, name, authors, price, **rest):
        '''rest的例子有：出版商、长度、标签、出版日期'''
        self.name = name
        self.authors = authors
        self.price = price  # 单位为美元
        self.__dict__.update(rest)

    def __str__(self):
        mylist = []
        ordered = OrderedDict(sorted(self.__dict__.items()))
        for i in ordered.keys():
            mylist.append('{}: {}'.format(i, ordered[i]))
            if i == 'price':
                mylist.append('$')
            mylist.append('\n')
            return ''.join(mylist)


class Prototype:
    def __init__(self):
        self.objects = dict()

    def register(self, identifier, obj):
        self.objects[identifier] = obj

    def unregister(self, identifier):
        del self.objects[identifier]

    def clone(self, identifier, **attr):
        found = self.objects.get(identifier)
        if not found:
            raise ValueError('Incorrect object identifier: {}'.format(identifier))
        obj = copy.deepcopy(found)
        obj.__dict__.update(attr)
        return obj


def main():
    b1 = Book('The C Programming Language', ('Brian W. Kernighan', 'Dennis M.Ritchie'),
              price=118, publisher='Prentice Hall', length=228, publication_date='1978-02-22',
              tags=('C', 'programming', 'algorithms', 'data structures'))
    prototype = Prototype()
    cid = 'k&r-first'
    prototype.register(cid, b1)
    b2 = prototype.clone(cid, name='The C Programming Language(ANSI)', price=48.99,
                         length=274, publication_date='1988-04-01', edition=2)
    for i in (b1, b2):
        print(i)
    print("ID b1 : {} != ID b2 : {}".format(id(b1), id(b2)))


if __name__ == '__main__':
    main()

"""
>>> python3 prototype.py
authors: ('Brian W. Kernighan', 'Dennis M. Ritchie')
length: 228
name: The C Programming Language
price: 118$
publication_date: 1978-02-22
publisher: Prentice Hall
tags: ('C', 'programming', 'algorithms', 'data structures')


authors: ('Brian W. Kernighan', 'Dennis M. Ritchie')
edition: 2
length: 274
name: The C Programming Language (ANSI)
price: 48.99$
publication_date: 1988-04-01
publisher: Prentice Hall
tags: ('C', 'programming', 'algorithms', 'data structures')

ID b1 : 140004970829304 != ID b2 : 140004970829472
"""