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I try to implement singleton pattern in python 2 but it doesn't work, but it works in python 3, someone help me explain why?

class Singleton:

    _instance = None

    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance

obj1 = Singleton()
obj2 = Singleton()

print(obj1)
print(obj2)

output:

<__main__.Singleton instance at 0x7fab8b625e60>
<__main__.Singleton instance at 0x7fab8b5e4710>
hiro protagonist
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1 Answers1

1

First of all in python2 you must be inherited from object to make __new__ magic works. Because old-style class doesn't have __new__ method at all.

So with added prints you will got:

>>> class Singleton:
...     _instance = None
...     def __new__(cls):
...         print('__new__')
...         if cls._instance is None:
...             print('create')
...             cls._instance = super().__new__(cls)
...         return cls._instance
... 
>>> obj1 = Singleton()
>>> obj2 = Singleton()
>>> 
>>> print(obj1)
<__main__.Singleton instance at 0x7f47dcccecb0>
>>> print(obj2)
<__main__.Singleton instance at 0x7f47dcccef80>

As you can see python2 doesn't call __new__ at all. It just calls empty __init__ in your case and creates two different objects.

Secondly, in python2 you need rewrite super() call as it was changed in python3. So corrected code will be:

>>> class Singleton(object):
...     _instance = None
...     def __new__(cls):
...         print('__new__')
...         if cls._instance is None:
...             print('create')
...             cls._instance = super(Singleton, cls).__new__(cls)
...         return cls._instance
... 
>>> obj1 = Singleton()
__new__
create
>>> obj2 = Singleton()
__new__
>>> 
>>> print(obj1)
<__main__.Singleton object at 0x7f47dccd9590>
>>> print(obj2)
<__main__.Singleton object at 0x7f47dccd9590>

For more information about singletons you can reed here: Creating a singleton in Python

James Brown
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