The Sororicide Antipattern

Don’t murder your parents or your siblings to get their attributes.

Composition is better than inheritance.”. This is a true statement. “Inheritance is bad.” Also true. I’m a well-known compositional extremist. There’s a great talk you can watch if I haven’t talked your ear off about it already.

Which is why I was extremely surprised in a recent conversation when my interlocutor said that while inheritance might be bad, composition is worse. Once I understood what they meant by “composition”, I was even more surprised to find that I agreed with this assertion.

Although inheritance is bad, it’s very important to understand why. In a high-level language like Python, with first-class runtime datatypes (i.e.: user defined classes that are objects), the computational difference between what we call “composition” and what we call “inheritance” is a matter of where we put a pointer: is it on a type or on an instance? The important distinction has to do with human factors.

First, a brief parable about real-life inheritance.


You find yourself in conversation with an indolent heiress-in-waiting. She complains of her boredom whiling away the time until the dowager countess finally leaves her her fortune.

“Inheritance is bad”, you opine. “It’s better to make your own way in life”.

“By George, you’re right!” she exclaims. You weren’t expecting such an enthusiastic reversal.

“Well,”, you sputter, “glad to see you are turning over a new leaf”.

She crosses the room to open a sturdy mahogany armoire, and draws forth a belt holstering a pistol and a menacing-looking sabre.

“Auntie has only the dwindling remnants of a legacy fortune. The real money has always been with my sister’s manufacturing concern. Why passively wait for Auntie to die, when I can murder my dear sister now, and take what is rightfully mine!”

Cinching the belt around her waist, she strides from the room animated and full of purpose, no longer indolent or in-waiting, but you feel less than satisfied with your advice.

It is, after all, important to understand what the problem with inheritance is.


The primary reason inheritance is bad is confusion between namespaces.

The most important role of code organization (division of code into files, modules, packages, subroutines, data structures, etc) is division of responsibility. In other words, Conway’s Law isn’t just an unfortunate accident of budgeting, but a fundamental property of software design.

For example, if we have a function called multiply(a, b) - its presence in our codebase suggests that if someone were to want to multiply two numbers together, it is multiply’s responsibility to know how to do so. If there’s a problem with multiplication, it’s the maintainers of multiply who need to go fix it.

And, with this responsibility comes authority over a specific scope within the code. So if we were to look at an implementation of multiply:

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def multiply(a, b):
    product = a * b
    return product

The maintainers of multiply get to decide what product means in the context of their function. It’s possible, in Python, for some other funciton to reach into multiply with frame objects and mangle the meaning of product between its assignment and return, but it’s generally understood that it’s none of your business what product is, and if you touch it, all bets are off about the correctness of multiply. More importantly, if the maintainers of multiply wanted to bind other names, or change around existing names, like so, in a subsequent version:

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def multiply(a, b):
    factor1 = a
    factor2 = b
    result = a * b
    return result

It is the maintainer of multiply’s job, not the caller of multiply, to make those decisions.

The same programmer may, at different times, be both a caller and a maintainer of multiply. However, they have to know which hat they’re wearing at any given time, so that they can know which stuff they’re still repsonsible for when they hand over multiply to be maintained by a different team.

It’s important to be able to forget about the internals of the local variables in the functions you call. Otherwise, abstractions give us no power: if you have to know the internals of everything you’re using, you can never build much beyond what’s already there, because you’ll be spending all your time trying to understand all the layers below it.

Classes complicate this process of forgetting somewhat. Properties of class instances “stick out”, and are visible to the callers. This can be powerful — and can be a great way to represent shared data structures — but this is exactly why we have the ._ convention in Python: if something starts with an underscore, and it’s not in a namespace you own, you shouldn’t mess with it. So: other._foo is not for you to touch, unless you’re maintaining type(other). self._foo is where you should put your own private state.

So if we have a class like this:

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class A(object):
    def __init__(self):
        self._note = "a note"

we all know that A()._note is off-limits.

But then what happens here?

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class B(A):
    def __init__(self):
        super().__init__()
        self._note = "private state for B()"

B()._note is also off limits for everyone but B, except... as it turns out, B doesn’t really own the namespace of self here, so it’s clashing with what A wants _note to mean. Even if, right now, we were to change it to _note2, the maintainer of A could, in any future release of A, add a new _note2 variable which conflicts with something B is using. A’s maintainers (rightfully) think they own self, B’s maintainers (reasonably) think that they do. This could continue all the way until we get to _note7, at which point it would explode violently.


So that’s why Inheritance is bad. It’s a bad way for two layers of a system to communicate because it leaves each layer nowhere to put its internal state that the other doesn’t need to know about. So what could be worse?

Let’s say we’ve convinced our junior programmer who wrote A that inheritance is a bad interface, and they should instead use the panacea that cures all inherited ills, composition. Great! Let’s just write a B that composes in an A in a nice clean way, instead of doing any gross inheritance:

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class Bprime(object):
    def __init__(self, a):
        for var in dir(a):
            setattr(self, var, getattr(a, var))

Uh oh. Looks like composition is worse than inheritance.


Let’s enumerate some of the issues with this “solution” to the problem of inheritance:

  • How do we know what attributes Bprime has?
  • How do we even know what type a is?
  • How is anyone ever going to grep for relevant methods in this code and have them come up in the right place?

We briefly reclaimed self for Bprime by removing the inheritance from A, but what Bprime does in __init__ to replace it is much worse. At least with normal, “vertical” inheritance, IDEs and code inspection tools can have some idea where your parents are and what methods they declare. We have to look aside to know what’s there, but at least it’s clear from the code’s structure where exactly we have to look aside to.

When faced with a class like Bprime though, what does one do? It’s just shredding apart some apparently totally unrelated object, there’s nearly no way for tooling to inspect this code to the point that they know where self.<something> comes from in a method defined on Bprime.

The goal of replacing inheritance with composition is to make it clear and easy to understand what code owns each attribute on self. Sometimes that clarity comes at the expense of a few extra keystrokes; an __init__ that copies over a few specific attributes, or a method that does nothing but forward a message, like def something(self): return self.other.something().

Automatic composition is just lateral inheritance. Magically auto-proxying all methods1, or auto-copying all attributes, saves a few keystrokes at the time some new code is created at the expense of hours of debugging when it is being maintained. If readability counts, we should never privilege the writer over the reader.


  1. It is left as an exercise for the reader why proxyForInterface is still a reasonably okay idea even in the face of this criticism.2 

  2. Although ironically it probably shouldn’t use inheritance as its interface. 

So You Want To Web A Twisted

I’m live-streaming a webinar on Twisted service architecture.

As a rehearsal for our upcoming tutorial at PyCon, Creating And Consuming Modern Web Services with Twisted, Moshe Zadka, we are doing a LIVE STREAM WEBINAR. You know, like the kids do, with the video games and such.

As the webinar gods demand, there is an event site for it, and there will be a live stream.

This is a practice run, so expect “alpha” quality content. There will be an IRC channel for audience participation, and the price of admission is good feedback.

See you there!

Make Time For Hope

Pandora hastened to replace the lid! but, alas! the whole contents of the jar had escaped, one thing only excepted, which lay at the bottom, and that was HOPE. So we see at this day, whatever evils are abroad, hope never entirely leaves us; and while we have THAT, no amount of other ills can make us completely wretched.

It’s been a rough couple of weeks, and it seems likely to continue to be so for quite some time. There are many real and terrible consequences of the mistake that America made in November, and ignoring them will not make them go away. We’ll all need to find a way to do our part.

It’s not just you — it’s legit hard to focus on work right now. This is especially true if, as many people in my community are, you are trying to motivate yourself to work on extracurricular, after-work projects that you used to find exciting, and instead find it hard to get out of bed in the morning.

I have no particular position of authority to advise you what to do about this situation, but I need to give a little pep talk to myself to get out of bed in the morning these days, so I figure I’d share my strategy with you. This is as much in the hope that I’ll follow it more closely myself as it is that it will be of use to you.

With that, here are some ideas.

It’s not over.

The feeling that nothing else is important any more, that everything must now be a life-or-death political struggle, is exhausting. Again, I don’t want to minimize the very real problems that are coming or the need to do something about them, but, life will go on. Remind yourself of that. If you were doing something important before, it’s still important. The rest of the world isn’t going away.

Make as much time for self-care as you need.

You’re not going to be of much use to anyone if you’re just a sobbing wreck all the time. Do whatever you can do to take care of yourself and don’t feel guilty about it. We’ll all do what we can, when we can.1

You need to put on your own oxygen mask first.

Make time, every day, for hope.

“You can stand anything for 10 seconds. Then you just start on a new 10 seconds.”

Every day, set aside some time — maybe 10 minutes, maybe an hour, maybe half the day, however much you can manage — where you’re going to just pretend everything is going to be OK.2

Once you’ve managed to securely fasten this self-deception in place, take the time to do the things you think are important. Of course, for my audience, “work on your cool open source code” is a safe bet for something you might want to do, but don’t make the mistake of always grimly setting your jaw and nose to the extracurricular grindstone; that would just be trading one set of world-weariness for another.

After convincing yourself that everything’s fine, spend time with your friends and family, make art, or heck, just enjoy a good movie. Don’t let the flavor of life turn to ash on your tongue.

Good night and good luck.

Thanks for reading. It’s going to be a long four years3; I wish you the best of luck living your life in the meanwhile.


  1. I should note that self-care includes just doing your work to financially support yourself. If you have a job that you don’t feel is meaningful but you need the wages to survive, that’s meaningful. It’s OK. Let yourself do it. Do a good job. Don’t get fired. 

  2. I know that there are people who are in desperate situations who can’t do this; if you’re an immigrant in illegal ICE or CBP detention, I’m (hopefully obviously) not talking to you. But, luckily, this is not yet the majority of the population. Most of us can, at least some of the time, afford to ignore the ongoing disaster. 

  3. Realistically, probably more like 20 months, once the Rs in congress realize that he’s completely destroyed their party’s credibility and get around to impeaching him for one of his numerous crimes. 

Sourceforge Update

Authenticate downloaded binaries from sourceforge a little more.

When I wrote my previous post about Sourceforge, things were looking pretty grim for the site; I (rightly, I think) slammed them for some pretty atrocious security practices.

I invited the SourceForge ops team to get in touch about it, and, to their credit, they did. Even better, they didn't ask for me to take down the article, or post some excuse; they said that they knew there were problems and they were working on a long-term plan to address them.

This week I received an update from said ops, saying:

We have converted many of our mirrors over to HTTPS and are actively working on the rest + gathering new ones. The converted ones happen to be our larger mirrors and are prioritized.

We have added support for HTTPS on the project web. New projects will automatically start using it. Old projects can switch over at their convenience as some of them may need to adjust it to properly work. More info here:

https://sourceforge.net/blog/introducing-https-for-project-websites/

Coincidentally, right after I received this email, I installed a macOS update, which means I needed to go back to Sourceforge to grab an update to my boot manager. This time, I didn't have to do any weird tricks to authenticate my download: the HTTPS project page took me to an HTTPS download page, which redirected me to an HTTPS mirror. Success!

(It sounds like there might still be some non-HTTPS mirrors in rotation right now, but I haven't seen one yet in my testing; for now, keep an eye out for that, just in case.)

If you host a project on Sourceforge, please go push that big "Switch to HTTPS" button. And thanks very much to the ops team at Sourceforge for taking these problems seriously and doing the hard work of upgrading their users' security.

Don’t Stop Tweeting On My Account

Context is everything; while some ideas can be whispered, others deserve a shout.

Shortly after my previous post, my good friend David Reid not-so-subtly subtweeted me for apparently yelling at everyone using a twitter thread to be quiet and stop expressing themselves. He pointed out:

This is the truth. There are, indeed, important, substantial essays being written on Twitter, in the form of threads. If I could direct your attention to one that’s probably a better use of your time than what I have to say here, this is a great example:

Moreover, although the twitter character limit can inhibit the expression of nuance, just having a blog is not a get-out-of-jail-free card for clumsy, hot takes:

I screwed this one up. I’m sorry.


The point I was trying to primarily focus on in that post is that a twitter thread demands a lot of attention, and that publishers exploiting that aspect of the medium in order to direct more attention to themselves1 are leveraging a limited resource2 and thereby externalizing their marketing costs3. Further, this idiom was invented by4, and has extensively been used by people who don’t really need any more attention than they already have.

If you’re an activist trying to draw attention to an important cause, or a writer trying to find your voice, and social media (or twitter threads specifically) has helped you do that, I am not trying to scold you for growing an audience on - and deriving creative energy from - your platform of choice. If you’re leveraging the focus-stealing power of twitter threads to draw attention to serious social issues, maybe you deserve that attention. Maybe in the face of such issues my convenience and comfort and focus are not paramount. And for people who really don’t want that distraction, the ‘unfollow’ button is, obviously, only a click away.

That’s not to say I think that relying on social media exclusively is a good idea for activists; far from it. I think recent political events have shown that a social media platform is often a knife that will turn in your hand. So I would encourage pretty much anyone trying to cultivate an audience to consider getting an independent web presence where you can host more durable and substantive collections of your thoughts, not because I don’t want you to annoy me, but because it gives you a measure of independence, and avoids a potentially destructive monoculture of social media. Given the mechanics of the technology, this is true even if you use a big hosted service for your long-form stuff, like Medium or Blogger; it’s not just about a big company having a hold on your stuff, but about how your work is presented based on the goals of the product presenting it.

However, the exact specifics of such a recommendation are an extremely complex set of topics, and not topics that I’m confident I’ve thought all the way through. There are dozens more problems with twitter threads for following long-form discussions and unintentionally misrepresenting complex points. Maybe they’re really serious, maybe not.

As far as where the long-form stuff should go, there are very good reasons to want to self-host things, and very good reasons why self-hosting is incredibly dangerous, especially for high-profile activists and intellectuals. There are really good reasons to engage with social media platforms and really good reasons to withdraw.

This is why I didn’t want to address this sort of usage of twitter threading; I didn’t want to dive into the sociopolitical implications of the social media ecosystem. At some point, you can expect a far longer post from me about the dynamics of social media, but it is going to take a serious effort to do it justice.


A final thought before I hopefully stop social-media-ing about social media for a while:

One of the criticisms that I received during this conversation, from David as well as others who contacted me privately, is that I’m criticizing Twitter from a level of remove; implying that since I’m not fully engaged with the medium I don’t really have the right (or perhaps the expertise) to be critical of it. I object to that.

In addition to my previously stated reasons for my reduced engagement - which mostly have to do with personal productivity and creative energy - I also have serious reservations about the political structure of social media. There’s a lot that’s good about it, but I think the incentive structures around it may mean that it is, ultimately, a fundamentally corrosive and corrupting force in society. At the very least, a social media platform is a tool which can be corrosive and corrupting and therefore needs to be used thoughtfully and intentionally to minimize the harm that it can do while retaining as many of its benefits as possible.

I don’t have time to fully explore the problems that I’m alluding to now5 but at this point if I wrote something like “social media platforms are slowly destroying liberal democracy”, I’m not even sure if I’d be exaggerating.

When I explain that I have these concerns, I’m often asked the obvious follow-up: if social media is so bad why don’t I just stop using it entirely?

The problem is, social media companies effectively control access to an enormous audience, which is now difficult to reach without their intermediation. I have friends, as we all probably do, that are hard for me to contact via other channels. An individual cannot effectively boycott a communication tool, and I am not even sure yet that “stop using it” is the right way to combat its problems.

So, I’m not going to stop communicating with my friends because I have concerns about the medium they prefer, and I’m also not going to stop thinking or writing about how to articulate and address those concerns. I think I have as much a right as anyone to do that.


  1. ... even if they’re not doing it on purpose ... 

  2. the reader’s attention 

  3. interrupting the reader repeatedly to get them to pay attention rather than posting stuff as a finished work, allowing the reader to make efficient use of their attention 

  4. I’m aware that many people outside of the white male tech nerd demographic - particularly women of color and the LGBTQ community - have made extensive use of the twitter thread for discussing substantive issues. But, as far as my limited research has shown (although the difficulty of doing such research is one of the problems with Twitter), Marc Andreessen was by far the earliest pioneer of the technique and by far its most prominent advocate. I’d be happy for a correction on this point, however. 

  5. The draft in progress, which I've been working on for a month, is already one of the longest posts I’ve ever written and it’s barely half done, if that.