What do you call a class not intended to be instantiated

Steven D'Aprano steven at REMOVE.THIS.cybersource.com.au
Mon Sep 22 11:32:39 CEST 2008

On Mon, 22 Sep 2008 10:11:58 +0200, Bruno Desthuilliers wrote:

> Steven D'Aprano a écrit :
>> I have a class which is not intended to be instantiated. Instead of
>> using the class to creating an instance and then operate on it, I use
>> the class directly, with classmethods. Essentially, the class is used
>> as a function that keeps state from one call to the next.
>> The problem is that I don't know what to call such a thing! "Abstract
>> class" isn't right, because that implies that you should subclass the
>> class and then instantiate the subclasses.
>> What do you call such a class?
> <nitpick>
> Err... A possible design smell ?-)
> </nitpick>
> More seriously: this looks quite like a singleton, which in Python is
> usually implemented way more simply using a module and plain functions.

I really don't know why everyone thinks I want a Singleton. I want to 
uncouple objects, not increase the coupling.

Consider a factory function:

def factory(x):  # a toy example
    alist = [x]
    def foo():
        return alist
    return foo

Now suppose we "instantiate" the factory (for lack of a better term):

>>> f1 = factory(0)
>>> f2 = factory(0)

Even though f1 and f2 have the same behaviour, they are obviously not the 
same object. And although both return a list [0], it is not the same list:

>>> f1() == f2() == [0]
>>> f1() is f2()

They have a (very little) amount of state, which is *not* shared:

>>> L = f1()
>>> L.append(1)
>>> f1()
[0, 1]
>>> f2()

But there's only a limited amount of state that functions carry around. I 
can give them more state like this:

>>> f1.attr = 'x'

but it isn't good enough if the function needs to refer to it's own 
state, because functions can only refer to themselves by name and the 
factory can't know what name the function will be bound to. 

As far as I know, the only objects that know how to refer to themselves 
no matter what name they have are classes and instances. And instances 
share at least some state, by virtue of having the same class.

(Pedants will argue that classes also share state, by virtue of having 
the same metaclass. Maybe so, but that's at a deep enough level that I 
don't care.)

I'm now leaning towards just having factory() instantiate the class and 
return the instance, instead of having to do metaclass chicanery. Because 
the class is built anew each time by the factory, two such instances 
aren't actually sharing the same class. I think that will reduce 
confusion all round (including mine!).

Hopefully now that I've explained what I want in more detail, it won't 
seem so bizarre. Factory functions do it all the time. Is there a name 
for this pattern?

Thanks to everyone who commented, your comments helped me reason out a 
better alternative to what I first suggested.


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