Skip to main content

Python's super() Abused as a Hook

There has been some recent-ish discussion on the python-dev mailing list about "fixing" the super built-in, which is used to access attributes of an object with lookup rules on the superclass of a given class. This is used for different things and in different ways, but the most common usage seems to be as follows:

class Foo(Bar):
def action(self):
super(Foo, self).action()
self.actionCalledOnFooInstance = True

This causes a call to Foo().action() to call the action method of the next class in the Method Resolution Order. Now, Bar.action might exist, or maybe Bar inherits from Baz and Baz.action() will be called. The point is, you don't have to know. The typical pattern here is the that we are looking for the superclass of the same class we are within (Foo) and call the same method we're already in (action), which is a repetition some people want to fix.

I propose that super() is not broken at all, or even failing, but simply that we are misusing it where anothing idiom may be more appropriate. The more appropriate idiom might be hooks. Sometimes our function using super() should actually be calling a hook and other times it should be the hook. In the example above, we want something to happen after the regular action() method is called, so we redefine the entire action() method to call the original and then execute our one little line. We require all the overhead and issues with super() largely for varients of this common use case. Instead, consider if we wrote the following:

class Foo(Bar):
def hook_after_action(self):
self.actionCalledOnFooInstance = True

Do you see how much simpler that is? The requirement here is that either super(Foo, self).action would call hook_after_action if it exists (or there could be a default no-op version) or some mechanism outside of the action() method might handle it, perhaps wrapping it up on request or at definition. Maybe a standard hook format could even be a candidate for brining into the language.

Hooks are very valuable concepts that are not used enough. We can save ourselves a lot of trouble by using them. There is a lot more I could say about them, but this post was mostly about their relation to the many use cases of super.

PS - Some of this can be known to relate to the concept called "Aspect-Oriented Programm" which has very little weight in the Python world, because here its so easy to do that it doesn't deserve a name and is reduced to simply wrapping functions or having hooks.


fumanchu said…
Within reason, yes. Classes which have a few, standard extension points are better written as hooks. But there are classes (such as adapters) where potentially every method needs to be extended. Adding a pre hook and a post hook to every method would be silly, polluting the namespace unnecessarily. The choice of when to add a hook method shouldn't be left to a whim.

One option is to isolate the hooks into a separate Hookset object, so that the space of hook names is at least isolated from the class namespace.
Ludvig said…
"...the super built-in, which is used to access attributes of an object with lookup rules on the superclass of a given class."

What you're saying here isn't entirely correct and much of the Python community is wrong here.

To lay it quick, what super() actually does is that it returns a proxy object of the attribute that would've been returned if the current function hadn't been called. The difference here is when it comes to MRO - multiple inheritance.

I suggest reading - it explains what I mean better than I can do quickly here.
Anonymous said…
This is wrong in a couple of ways. First, you do need to know whether baz.action is being called or bar.action is being called. Things are going to be screwed up, and you're going to be debugging, and it's going to be important that you know exactly which routine was called when.

Secondly, it's going to be the case that you want both of baz.action and bar.action to be called. The hook approach won't work. As soon as you declare your hook, it overrides the hook in the superclass that you wanted to have called. So, your hook needs to call super(foo, self).hook().

Popular posts from this blog

The Insidiousness of The Slow Solution

In software development, slow solutions can be worse than no progress at all. I'll even say its usually worse and if you find yourself making slow progress on a problem, consider stopping while you're a head.

Its easy to see why fast progress is better: either you solve the problem or you prove a proposed solution wrong and find a better one. Even a total standstill in pushing forward on a task or a bug or a request can force you to seek out new information or a second opinion.

Slow solutions, on the other hand, is kind of sneaky. Its insidious. Slow solution is related the Sunk Cost Fallacy, but maybe worse. Slow solutions have you constantly dripping more of your time, energy, and hope into a path that's still unproven, constantly digging a hole. Slow solutions are deceptive, because they still do offer real progress. It is hard to justify abandoning it or trying another route, because it is "working", technically.

We tend to romanticize the late night hacking…

Finding "One Game A Month"

I was really excited about the One Game A Month challenge as soon as I heard about it.
For about two years I've struggled in fits and starts to make my way into game development. This hasn't been productive in any of the ways I hoped when I started. Its really difficult to be fairly experienced as a developer, which I believe I am in my day job as a web developer, while struggling really hard at an area in which your experience just doesn't exist.
Its like being a pilot who doesn't know how to drive.

But this challenge provided a new breath to this little hobby of mine. It gave me a scaffolding to experiment, to learn, to reflect on finished projects. I had spent far too much time on game projects that stretched on far past their exciting phases, bogged down by bad decisions and regret.
And it has worked.
I have a lot to learn. I have a lot of experience to gain through trial and error and mistake and discovery. I have a lot of fun to be had making more small games t…

On Pruning Your Passions

We live in a hobby-rich world. There is no shortage of pastimes to grow a passion for. There is a shortage of one thing: time to indulge those passions. If you're someone who pours your heart into that one thing that makes your life worthwhile, that's a great deal. But, what if you've got no shortage of interests that draw your attention and you realize you will never have the time for all of them?

If I look at all the things I'd love to do with my life as a rose bush I'm tending, I realize that careful pruning is essential for the best outcome. This is a hard lesson to learn, because it can mean cutting beautiful flowers and watching the petals fall to the ground to wither. It has to be done.

I have a full time job that takes a lot of my mental energy. I have a wife and a son and family time is very important in my house. I try to read more, and I want to keep up with new developments in my career, and I'm trying to make time for simple, intentional relaxing t…