如何實(shí)現(xiàn)Python中的單例

1.重寫Class的new方法

#__new__函數(shù)就是class在創(chuàng)建對(duì)象時(shí)會(huì)調(diào)用的方法
#通過重寫__new__函數(shù)可以控制只創(chuàng)建一個(gè)對(duì)象
#子類可以通過重寫__new__函數(shù)
class Singleton(object):
    _instance = None
    def __new__(class_, *args, **kwargs):
        if not isinstance(class_._instance, class_):
            class_._instance = object.__new__(class_, *args, **kwargs)
        return class_._instance

2.指定metaclass

#通過metaclass控制class的__new__函數(shù)
#子類除非指定別的metaclass,否則無(wú)法改變單例行為
class Singleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        else:
           #這段代碼的作用是每次都調(diào)用class的init函數(shù)
            cls._instances[cls].__init__(*args, **kwargs)
        return cls._instances[cls]

#Python2
class MyClass(BaseClass):
    __metaclass__ = Singleton

#Python3
class MyClass(BaseClass, metaclass=Singleton):
    pass

PS:上面的寫法都沒有加線程鎖,如果考慮在多線程的環(huán)境下使用需要加線程鎖

import functools
import threading

lock = threading.Lock()


def synchronized(lock):
    """ Synchronization decorator """
    def wrapper(f):
        @functools.wraps(f)
        def inner_wrapper(*args, **kw):
            with lock:
                return f(*args, **kw)
        return inner_wrapper
    return wrapper


class Singleton(type):
    _instances = {}

    @synchronized(lock)
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]


class SingletonClass(metaclass=Singleton):
    pass

PS: 在Python中,import默認(rèn)只會(huì)導(dǎo)入一次,除非使用reload函數(shù)刷新,否則每個(gè)module只會(huì)導(dǎo)入一次,這樣的話,如果導(dǎo)入一個(gè)變量,則這個(gè)變量就相當(dāng)于單例