[FIXED] Python singleton acting strange

Issue

I've found one of most popular approach to create singleton classes, using _instance and overriding __new__ method. Here it is:

class Singleton:
    _instance = None

    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super().__new__(cls)
        return cls._instance

But it acting weird when I try to use it this way:

class Child(Singleton):
    def __init__(self):
        self.a = random.randint(10, 1000)

x = Child()
y = Child()
print(x.__dict__)        # {'a': 74}
print(y.__dict__)        # {'a': 74}  - not weird
print(Child().__dict__)  # {'a': 222} - weird (for me)

Can someone explain why it happens?

PS: I'm using python 3.10

Solution

An answer

In order to create an object, Python first calls __new__ on its class, then calls __init__ on the object.

When you "create" an instance of Child, Python will call Singleton.__new__ and then Child.__init__. But Singleton.__new__ always returns the same object, _instance, so Child.__init__ will always re-initialise the same object.

x and y contain the same object, because all Childs are the same object. That's why x.__dict__ and y.__dict__ are the same. But when you call Child the third time, it reinitialises the object, changing its dict. print(Child().__dict__) gives a different value because it calls Child before printing.

It's not that the third call to Child produces a different object; rather that the object itself changes.

Some notes on singletons

You shouldn't use __init__ if you're implementing a singleton in this way, because it will be called every time Child is called and reinitialise the object. If you need to initialise values of the singleton, you can do so in __new__, within the if statement, so that the initialisation is only done once.

Also consider why you need to create a subclass of a singleton. A singleton should only ever have one member, so if there are any instances of Child, it must be that all Singletons are Childs, and so Singleton and Child might as well be merged into one class.

Answered By - Anonymous12358