模块在一个Python应用程序中只有一份，它本身就是单例的，将所需要的属性和方法，直接暴露在模块中的全局变量和方法，也可以实现单例。

Python单例模式的4种实现

# !/usr/bin/env python
# -*- coding:utf-8 -*-
  
print '----------------------方法1--------------------------'
# 方法1, 实现__new__方法
# 并在将一个类的实例绑定到类变量_instance上,
# 如果cls._instance为None说明该类还没有实例化过,实例化该类并返回
# 如果cls._instance不为None,直接返回cls._instance

class Singleton(object):
    def __new__(cls, *args, **kw):
        if not hasattr(cls, '_instance'):
            new = super(Singleton, cls)
            cls._instance = new.__new__(cls, *args, **kw)
        return cls._instance

class MyClass(Singleton):
    a = 1

one = MyClass()
two = MyClass()

two.a = 3
print one.a         # 3

# one和two完全相同,可以用id(), ==, is检测
print id(one)       # 140382102515600
print id(two)       # 140382102515600
print one == two    # True
print one is two    # True


print '----------------------方法2--------------------------'
# 方法2,共享属性;所谓单例就是所有引用(实例、对象)拥有相同的状态(属性)和行为(方法)
# 同一个类的所有实例天然拥有相同的行为(方法),
# 只需要保证同一个类的所有实例具有相同的状态(属性)即可
# 所有实例共享属性的最简单最直接的方法就是__dict__属性指向(引用)同一个字典(dict)
# 可参看:http://code.activestate.com/recipes/66531/
class Singleton2(object):
    _state = {}
    def __new__(cls, *args, **kw):
        ob = super(Singleton2, cls).__new__(cls, *args, **kw)
        ob.__dict__ = cls._state
        return ob

class MyClass2(Singleton2):
    a = 1

one = MyClass2()
two = MyClass2()

# one和two是两个不同的对象,id, ==, is对比结果可看出
two.a = 3
print one.a         # 3

print id(one)       # 140630201217168
print id(two)       # 140630201217232
print one == two    # False
print one is two    # False

# 但是one和two具有相同的（同一个__dict__属性）,见:
print id(one.__dict__)      # 140630201186296
print id(two.__dict__)      # 140630201186296


print '----------------------方法3--------------------------'
# 方法3:本质上是方法1的升级（或者说高级）版
# 使用__metaclass__（元类）的高级python用法
class Singleton3(type):
    def __init__(cls, name, bases, dict):
        super(Singleton3, cls).__init__(name, bases, dict)
        cls._instance = None
        
    def __call__(cls, *args, **kw):
        if cls._instance is None:
            cls._instance = super(Singleton3, cls).__call__(*args, **kw)
        return cls._instance

class MyClass3(object):
    __metaclass__ = Singleton3

one = MyClass3()
two = MyClass3()

two.a = 3
print one.a         # 3

print id(one)       # 140630201217424
print id(two)       # 140630201217424
print one == two    # True
print one is two    # True


print '----------------------方法4--------------------------'
# 方法4:也是方法1的升级（高级）版本,
# 使用装饰器(decorator),
# 这是一种更pythonic,更elegant的方法,
# 单例类本身根本不知道自己是单例的,因为他本身(自己的代码)并不是单例的
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 MyClass4(object):
    a = 1
    def __init__(self, x=0):
        self.x = x

one = MyClass4()
two = MyClass4()

two.a = 3
print one.a         # 3

print id(one)       # 140630201217168
print id(two)       # 140630201217168
print one == two    # True
print one is two    # True

one.x = 1
print one.x         # 1
print two.x         # 1

参考推荐：

Is there a simple, elegant way to define Singletons in Python? （Stack Overflow）