python单例模式的几种写法

---

一、装饰器模式

def singleton(cls, *args, **kwargs):
    instance = {}
    def _singleton():
        if cls not in instance:
            instance[cls] = cls(*args, **kwargs)
        return instance[cls]
    return _sigleton

@singleton
class Test(object):
    a = 1
    
test = Test()
test1 = Test()
print(id(test) == id(test1))
            
>>> True

二、使用new方法

class Singleton(object):
    def __new__(cls, *args, **kwargs):
        if not hasattr(cls, '_instance'):
            cls._instance = super(Singleton, cls).__new__(cls, *args, **kargs)
        return cls._instance
        
class Test(Singleton):
    a = 1
    
test = Test()
test1 = Test()
print(id(test) == id(test1))
>>> True

三、共享dict方法

class Singleton(object):
    _state = {}
    def __new__(cls, *args, **kwargs):
        obj = super(Singleton, cls).__new__(cls, *args, **kwargs)
        obj.__dict__ = obj._state
        return obj
        
class Test(Singleton):
    a = 1
    
test = Test()
test1 = Test()
print(id(test) == id(test1))
print(id(test.__dict__) == id(test1.__dict__))
>>> False
>>> True

四、import导入

# a.py
class Test(object):
    a = 1
test = Test()

# b.py
from a import test
print(id(test))

# c.py
from a import test
print(id(test))