Python – Singleton

Singleton in Python

import functools

def singleton(cls):
    """ Use class as singleton. """

    cls.__new_original__ = cls.__new__

    @functools.wraps(cls.__new__)
    def singleton_new(cls, *args, **kw):
        it =  cls.__dict__.get('__it__')
        if it:
            return it
        cls.__it__ = it = cls.__new_original__(cls, *args, **kw)
        it.__init_original__(*args, **kw)
        return it

    cls.__new__ = singleton_new
    cls.__init_original__ = cls.__init__
    cls.__init__ = object.__init__

    return cls

Sample use

@singleton
class Foo:
    def __new__(cls):
        cls.x = 10
        return object.__new__(cls)

    def __init__(self):
        assert self.x == 10
        self.x = 15

assert Foo().x == 15
Foo().x = 20
assert Foo().x == 20