How to always use the same instance of a class in Python?

Question

I am using the following solution to maintain a list of classes instances: https://stackoverflow.com/a/12102163

Now I want to use that list to make sure that there is always only one instance of the class. Renewed initializations of the class should return the one existing instance.

The code I have is:

class MyClass:

    instances = []

    def __init__(self):
        if not MyClass.instances:
            self.data=1234
        else:
            self = MyClass.instances[0]

So:

>> a=MyClass()
>> a.data
1234

And b=MyClass() should return the same instance as a. This is not working. What is wrong with my code?

EDIT: OK, so it turns out I am looking for the singleton pattern, but couldn't recreate it myself. Can anyone please explain why my code does not work?

related: [Is there a simple, elegant way to define Singletons in Python?](http://stackoverflow.com/questions/31875/is-there-a-simple-elegant-way-to-define-singletons-in-python) — jfs, Apr 17 '15 at 11:29
To answer the question at the end: what's wrong with your code, is that `self` is just a local variable in `__init__`. Assigning a value to it doesn't change what value is returned from the call to `MySelf()`. — Steve Jessop, Apr 17 '15 at 11:29
Using singletons (and particularly using singletons in Pythong) seem to be... _discouraged_ lately... See http://stackoverflow.com/questions/137975/what-is-so-bad-about-singletons and http://stackoverflow.com/questions/1318406/why-is-the-borg-pattern-better-than-the-singleton-pattern-in-python — BorrajaX, Apr 17 '15 at 11:32

score 1 · Answer 1 · answered Apr 17 '15 at 11:38

1

Going on your code line and your style. You can make following modifications:-

class MyClass:
    instance = None
    def __init__(self):
        self.data=1234
        MyClass.instance = self

def get_myclass():
    if MyClass.instance:
        return MyClass.instance
    return MyClass()

get_myclass would be a wrapper function for creating class objects. Trying the code.

>>> import instance
>>> a=instance.get_myclass()
>>> b=instance.get_myclass()
>>> a is b
True

answered Apr 17 '15 at 11:38

Barun Sharma

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I have code calling `MyClass()` which I do not want to change to `get_myclass`. Is there a way to use `MyClass` only? – boadescriptor Apr 17 '15 at 12:00

Doug R. · Answer 2 · 2015-05-28T19:36:21.590

Here's an example of the Singleton metaclass recipe. This example is from Ch. 9 of The Python Cookbook.

class Singleton(type):
    def __init__(self, *args, **kwargs):
        self.__instance = None
        super().__init__(*args, **kwargs)

    def __call__(self, *args, **kwargs):
        if self.__instance is None:
            self.__instance = super().__call__(*args, **kwargs)
            return self.__instance
        else:
            return self.__instance

# Example
class Spam(metaclass=Singleton):
    def __init__(self):
        print('Creating Spam')


if __name__ == '__main__':
    a = Spam()
    b = Spam()
    assert a is b

How to always use the same instance of a class in Python?

2 Answers2