摘要：最近在學(xué)習(xí)設(shè)計(jì)模式而開(kāi)發(fā)的語(yǔ)言中比較中意的就是了在這里總結(jié)下設(shè)計(jì)模式一般分為三大類構(gòu)造型結(jié)構(gòu)型行為型先從創(chuàng)建型設(shè)計(jì)模式開(kāi)始創(chuàng)建型設(shè)計(jì)模式包括單例模式抽象工廠模式工廠方法模式生成器模式惰性初始化模式對(duì)象池模式原型模式單例模式單例模式的定義是保

最近在學(xué)習(xí)設(shè)計(jì)模式,而開(kāi)發(fā)的語(yǔ)言中比較中意的就是python了,在這里總結(jié)下.

設(shè)計(jì)模式一般分為三大類:構(gòu)造型,結(jié)構(gòu)型,行為型

先從創(chuàng)建型設(shè)計(jì)模式開(kāi)始,創(chuàng)建型設(shè)計(jì)模式包括:單例模式,抽象工廠模式,工廠方法模式,生成器模式,惰性初始化模式,對(duì)象池模式,原型模式.

單例模式的定義是:保證一個(gè)類僅有一個(gè)實(shí)例,并提供一個(gè)它的全局訪問(wèn)點(diǎn).
先來(lái)看看14年前(沒(méi)寫錯(cuò))的前輩介紹的單例模式例程

from __future__ import print_function


class Borg:
    __shared_state = {}

    def __init__(self):
        self.__dict__ = self.__shared_state
        self.state = "Init"

    def __str__(self):
        return self.state


class YourBorg(Borg):
    pass


if __name__ == "__main__":
    rm1 = Borg()
    rm2 = Borg()

    rm1.state = "Idle"
    rm2.state = "Running"

    print("rm1:{0}".format(rm1))
    print("rm2:{0}".format(rm2))

    rm2.state = "Zombie"

    print("rm1:{0}".format(rm1))
    print("rm2:{0}".format(rm2))

    print("rm1 id:{0}".format(id(rm1)))
    print("rm2 id:{0}".format(id(rm2)))

    rm3 = YourBorg()

    print("rm1:{0}".format(rm1))
    print("rm2:{0}".format(rm2))
    print("rm3:{0}".format(rm3))


# Output
# rm1:Running
# rm2:Running
# rm1:Zombie
# rm2:Zombie
# rm1 id:140609170593696
# rm2 id:140609170593624
# rm1:Init
# rm2:Init
# rm3:Init

簡(jiǎn)單解釋下,需要get的點(diǎn)是下面這段代碼

    __shared_state = {}
    
    def __init__(self):
        self.__dict__ = self.__shared_state
        self.state = "Init"

self.__dict__是對(duì)象的字典表示.將對(duì)象的屬性設(shè)為全局靜態(tài)變量.
根據(jù)輸出結(jié)果,rm1和rm2儼然是一個(gè)實(shí)例.然而打印出來(lái)的rm1,rm2的變量id是不一致的,所以rm1,rm2并不是同一個(gè)實(shí)例.但是卻有同樣的狀態(tài)和行為.但從結(jié)果上來(lái)看,確實(shí)實(shí)現(xiàn)了單例模式的要求.(雖然有種走捷徑的感覺(jué))

下面看看另一個(gè)版本的,其實(shí)就是換個(gè)形式,原理還是建多個(gè)實(shí)例,表現(xiàn)一致.其他部分的代碼和上面的一致.

class Borg(object):
    __state = {}

    def __new__(cls, *p, **k):
        self = object.__new__(cls, *p, **k)
        self.__dict__ = cls.__state
        return self

    def __init__(self):
        self.state = "Init"

單例模式的創(chuàng)建有很多種方式.
這里有討論鏈接描述

升級(jí)版,通過(guò)__metaclass__實(shí)現(xiàn).這個(gè)版本中同一個(gè)id說(shuō)明是同一個(gè)對(duì)象.

class Singleton(type):

    def __init__(self, name, bases, dict):
        super(Singleton, self).__init__(name, bases, dict)
        self._instance = None

    def __call__(self, *args, **kw):
        if self._instance is None:
            self._instance = super(Singleton, self).__call__(*args, **kw)
        return self._instance


class MyClass(object):
    __metaclass__ = Singleton


one = MyClass()
two = MyClass()

two.a = 3
print one.a
# 3
print id(one)
# 139798461922256

print id(two)
# 139798461922256

print one == two
# True

print one is two
# True

還可以通過(guò)裝飾器來(lái)實(shí)現(xiàn).這是第一個(gè)方法的高級(jí)版本,更pythonic,更elegant的方法.
這個(gè)例子中,單例類本身不知道自己是單例的,應(yīng)為他本身(自己的代碼)就不是單例的.

def singleton(cls, *args, **kw):
    instances = {}

    def _singleton():
        if cls not in instances:
            instances[cls] = cls(*args, **kw)
        return instances[cls]
    return _singleton


@singleton
class MyClass(object):
    a = 1

    def __init__(self, x=0):
        self.x = x


one = MyClass()
two = MyClass()

two.a = 3
print one.a
# 3

print id(one)
# 140630714076048

print id(two)
# 140630714076048

print one == two
# True

print one is two
# True

one.x = 1

print one.x
# 1

print two.x
# 1

好東西留到最后,來(lái)個(gè)超級(jí)無(wú)敵版的

class Singleton:
    """
    A non-thread-safe helper class to ease Implementing singletons.
    This should be used as a decorator -- not a metaclass -- to the
    class that should be singleton.

    The decorated class can define one `__init__` function that
    takes onelythe `self` argument. Other than that, there are no
    restrictions that apply to the decorated class.

    To get the singleton instances, use the `instances` method. Trying
    to use `__call__` will result in a `TypeError` being raised.

    Limitations: The decorated class cannot be inherited from.
    """

    def __init__(self, decorated):
        self._decorated = decorated

    def instance(self):
        """
        Return the singleton instance. Upon this first call, it creates
        a new instance of decorated class and calls its `__init__` method.
        On all subsequent calls, the already created instance is returned.
        """
        try:
            return self._instance
        except AttributeError:
            self._instance = self._decorated()
            return self._instance

    def __call__(self):
        raise TypeError("Singletons must be accessed through `instance()`.")

    def __instancecheck(self, inst):
        return isinstance(inst, self._decorated)


@Singleton
class Foo:
    def __init__(self):
        print "Foo created"


# f = Foo()
# TypeError: Singletons must be accessed through `instance()`.

f = Foo.instance()
g = Foo.instance()
# Foo created

print f is g
# True

關(guān)于python的單例模式,各家說(shuō)法不一.可以根據(jù)不同的需求,使用不同的方式,適合的才是最好的.

參考文字:
https://zh.wikipedia.org/wiki/%E8%AE%BE%E8%AE%A1%E6%A8%A1%E5%BC%8F_(%E8%AE%A1%E7%AE%97%E6%9C%BA)

http://ginstrom.com/scribbles/2007/10/08/design-patterns-python-style/

http://code.activestate.com/recipes/66531/

http://blog.csdn.net/ghostfromheaven/article/details/7671853

http://stackoverflow.com/questions/31875/is-there-a-simple-elegant-way-to-define-singletons-in-python/31887#31887