Question

I have come across this singleton implementation here: http://blog.amir.rachum.com/post/21850841339/implementing-the-singleton-pattern-in-python in the first reply.

def singleton(cls):
    return cls()

@singleton
class Foo(object):
    def bar(self):
        pass

if __name__ == '__main__':

    print id(Foo)
    print id(Foo) 

But I don’t understand the inner workings, the decorator returns a class instance, but why the same instance every time ?

Answer 1

You can rewrite that code to

class Foo(object):
    pass

Foo = singleton(Foo)
# which is
Foo = Foo()

So here the name of the class is replaced by an instantiation of it. A bit cheesy in my opinion, especially since you can still create new objects of the same class by using Foo.__class__ and you are messing with the naming schema.