python singleton class decorator

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.

Answer 2

The singleton does that by holding internal state. This state here would probably be an instance of the class. The decorator can be something arbitrary.

Have a look at this:

http://hairysun.com/downloads/DecoratorHandout.pdf

class Decorator(object):
    # in __init__ set up state
    def __call__(self, function):
        @functools.wraps(function)
        def wrapper(*args, **kw): # 1.
            print "before func"
            result = function(*args, **kw) # 2.
            print "after func"
            return result
        return wrapper # 3.

>>> decorator2 = Decorator()
>>> @decorator2
... def nothing(): pass

The decorator is essentially a function that

1. Defines a function
2. That calls the function that you passed in
3. Returns the newly 'wrapped' function to be called later

The surrounding class (here: the decorator) could do something like this:

class Singleton(object):
     def __init__(self):
        self.instance = None

     def __call__(self, function):
         @functools.wraps(function)
         def wrapper(*args, **kw):
             if self.instance is None:
                self.instance = function(*args, **kw)
             return self.instance
         return wrapper

I did not run the code, but I assume this is in general how it works. If there is no instance available create one. If one is available, don't create a new one - return the single old one instead. One might probably want to check for other properties of the callable before using this in production.