摘要：最近在學習設計模式而開發的語言中比較中意的就是了在這里總結下設計模式一般分為三大類構造型結構型行為型先從創建型設計模式開始創建型設計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

最近在學習設計模式,而開發的語言中比較中意的就是python了,在這里總結下.

設計模式一般分為三大類:構造型,結構型,行為型

先從創建型設計模式開始,創建型設計模式包括: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,對象池模式,原型模式.

單例模式的定義是:保證一個類僅有一個實例,并提供一個它的全局訪問點.
先來看看14年前(沒寫錯)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

簡單解釋下,需要get的點是下面這段代碼

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

self.__dict__是對象的字典表示.將對象的屬性設為全局靜態變量.
根據輸出結果,rm1和rm2儼然是一個實例.然而打印出來的rm1,rm2的變量id是不一致的,所以rm1,rm2并不是同一個實例.但是卻有同樣的狀態和行為.但從結果上來看,確實實現了單例模式的要求.(雖然有種走捷徑的感覺)

下面看看另一個版本的,其實就是換個形式,原理還是建多個實例,表現一致.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"

單例模式的創建有很多種方式.
這里有討論鏈接描述

升級版,通過__metaclass__實現.這個版本中同一個id說明是同一個對象.

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

還可以通過裝飾器來實現.這是第一個方法的高級版本,更pythonic,更elegant的方法.
這個例子中,單例類本身不知道自己是單例的,應為他本身(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,來個超級無敵版的

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

關于python的單例模式,各家說法不一.可以根據不同的需求,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