使用python中的构造函数限制在单例类中创建对象

Posted 2021-04-08

我正在为python中的一个需求编写单例类。我可以意识到，我无法限制该类的对象创建，如果某个客户端在使用单例类方法之前首先使用构造函数进行调用（意味着返回该类的唯一实例的方法）。

我的意思是什么？让我用一些代码片段解释一下。考虑我有下面的一个示例：

import threading
class MyClass:

    __instance = None

    def __init__(self):

        if self.__instance:
            raise ValueError("Already exist")


    @classmethod
    def getInstance(cls):

        lock = threading.Lock()

        with lock:
            if cls.__instance == None:
                cls.__instance = MyClass()
            return cls.__instance

    def printObjInfo(self,obj):
        print(id(obj))


if __name__ == '__main__':

    ob4 = MyClass()
    ob4.printObjInfo(ob4)

    obj1 = MyClass.getInstance()
    obj1.printObjInfo(obj1)

    obj2 = MyClass.getInstance()
    obj2.printObjInfo(obj2)

    # Create a thread
    obj3 = MyClass.getInstance()
    obj3.printObjInfo(obj3)
    t1 = threading.Thread(target=obj3.printObjInfo, args=(obj3))

如果我运行上面的代码片段，我会得到如下结果：

44824952 - Object created by constructor.
44826240 - Object created by getInstance() method.
44826240 - Object created by getInstance() method.
44826240 - Object created by getInstance() method.

有一点需要注意 - 如果有人在调用getInstance（）方法之后调用构造函数，那么我们可以很容易地限制其他对象的创建。但如果它首先被调用，那么我们将无法控制它。

现在的问题是 - 1）我不能在__init __（）中放任何进一步的条件，没有人调用它或2）不能使我的构造函数私有 - 我可以吗？

我在这里找到了一些参考 - Program to restrict object creation in Python但不确定，我们如何限制第一个对象创建本身。有没有更好的方法让你这样做？

有什么想法或参考？

谢谢。

答案

您可以改为覆盖__new__：

class Singleton:

    _instance = None

    def __init__(self, arg):
        self.arg = arg

    def __new__(cls, arg):
        if cls._instance is None:
            cls._instance = object.__new__(cls)
            return cls._instance

        else:
            return cls._instance

print(Singleton(None))
print(Singleton(None))
print(Singleton(None))

输出：

<__main__.Singleton object at 0x1207fa550>
<__main__.Singleton object at 0x1207fa550>
<__main__.Singleton object at 0x1207fa550>

这样做的好处是，有一个统一的界面可用于创建和获取Singleton的单个实例：构造函数。

请注意，除非您编写自己的C扩展或类似的东西，否则您将永远无法在Python中创建真正的单例：

print(object.__new__(Singleton))
print(object.__new__(Singleton))
print(object.__new__(Singleton))

输出：

<__main__.Singleton object at 0x120806ac8>
<__main__.Singleton object at 0x1207d5b38>
<__main__.Singleton object at 0x1207d5198>

另一答案

基于gmds的答案的线程安全单例版本

from multiprocessing.dummy import Pool as ThreadPool 
import threading
import time

class ThreadSafeSingleton:
    __instance = None
    __lock = threading.Lock()

    def __init__(self, *args, **kwargs):
        self.args = args
        self.kwargs = kwargs

    def __str__(self):
        return 'id: {}, args: {}, kwargs: {}'.format(id(self), self.args, self.kwargs)

    def __new__(cls, *args, **kwargs):
        with cls.__lock:
            if cls.__instance is None:
                # sleep just simulate heavy class initialization  !!
                # process under concurrency circumstance          !!
                time.sleep(1)
                print('created')
                cls.__instance = super(ThreadSafeSingleton, cls).__new__(cls)
            return cls.__instance


class ThreadUnsafeSingleton:
    __instance = None

    def __init__(self, *args, **kwargs):
        self.args = args
        self.kwargs = kwargs

    def __str__(self):
        return 'id: {}, args: {}, kwargs: {}'.format(id(self), self.args, self.kwargs)

    def __new__(cls, *args, **kwargs):
        if cls.__instance is None:
            time.sleep(1)
            print('created')
            cls.__instance = super(ThreadUnsafeSingleton, cls).__new__(cls)
        return cls.__instance


def create_safe(*args, **kwargs):
    obj = ThreadSafeSingleton(*args, **kwargs)
    print(obj)


def create_unsafe(*args, **kwargs):
    obj = ThreadUnsafeSingleton(*args, **kwargs)
    print(obj)


if __name__ == '__main__':
    pool = ThreadPool(4)
    print('---- test thread safe singleton  ----')
    pool.map(create_safe, range(10))

    print('
---- test thread unsafe singleton  ----')
    pool.map(create_unsafe, range(10))

输出：

---- test thread safe singleton  ----
created
id: 4473136352, args: (0,), kwargs: {}
id: 4473136352, args: (1,), kwargs: {}
id: 4473136352, args: (2,), kwargs: {}
id: 4473136352, args: (4,), kwargs: {}
id: 4473136352, args: (5,), kwargs: {}
id: 4473136352, args: (3,), kwargs: {}
id: 4473136352, args: (6,), kwargs: {}
id: 4473136352, args: (7,), kwargs: {}
id: 4473136352, args: (8,), kwargs: {}
id: 4473136352, args: (9,), kwargs: {}

---- test thread unsafe singleton  ----
created
id: 4473136968, args: (0,), kwargs: {}
created
created
created
id: 4473136968, args: (4,), kwargs: {}
id: 4473137024, args: (2,), kwargs: {}
id: 4473137080, args: (1,), kwargs: {}
id: 4473137136, args: (3,), kwargs: {}
id: 4473137136, args: (5,), kwargs: {}
id: 4473137136, args: (7,), kwargs: {}
id: 4473137136, args: (6,), kwargs: {}
id: 4473137136, args: (8,), kwargs: {}
id: 4473137136, args: (9,), kwargs: {}