Python’s Innards: for my wife

The other day the best wife I’ll ever have had trouble sleeping and asked me to tell her something to put her to sleep. Since she’s not quite a hacker, I figured some discussion of what I usually write about may do the trick (okay, maybe ‘not quite a hacker’ is an understatement, she’s an un-hacker-hippie married to a hacker and it actually works out great thanks for asking). I don’t think this article is likely to be actually useful to the usual readers of this blog (unless you have a less-computer-initiated spouse with trouble sleeping), but I hope maybe this particular bedtime story might entertain some of you.

Yaniv: What is my blog about… Let’s start at the beginning. See, a computer is a machine, just a machine, you know, like one with gears and levers and stuff. Like all machines, it’s not smart, but unlike many machines, it’s has lots of pieces and is very very fast. Think about the musical boxes we saw today at the Museum’s souvenir store. They are also machines, but smaller and slower. When you twist the ratchet lever the pin cylinder turns around to make music. But even though the music is great, it isn’t a ‘smart’ machine, it just plays the music it was built to play. A machine with such and such a pin cylinder will play Mozart, and a machine with a different pin cylinder will play Bach. All right?

Inbal: An encouraging nod.

Yaniv: So think about a music box with replacable cylinder. Instead of a cylinder with small pins it could have a… eh… a punched card! A long strip of plastic or cardboard you insert into machine, and this strip has some holes punched in it. When the card is fed into the machine, the music box’s comb’s teeth encounter these holes like the encounter the pins and music is played. Computers are very much like that, only they don’t play music, they punch holes in a fresh card according to the card that was inserted. That’s how you program them, and that’s how they answer. You insert a card with holes arranged in just the right way, and you read the answer encoded as holes in the card the spew out. Not magic, just how the internals of the computer are built, like maybe an abacus controlled with cards. If you restart it ten times and feed it the same card, it will give the same answer each time. It’s a machine.

Inbal: Is that why people like you always tell people like me to restart the computer?

Yaniv: Yes, quite. We want to know exactly what kind of punched cards were fed into the computer from the moment it was last started, so we know what state it is in, so maybe we can actually figure out why it’s behaving like it is. Another encouraging nod, and wide open eyes. My wife shows general interest in my work, but never anything as low-level as this shit. I was wondering how far can we go on this topic. The thing is, you obviously know computers don’t really use punched cards anymore. Many years ago smart people figured out more elegant ways to talk with a computer, so we feed it with keyboards and USB sticks and wifi and we see the output on a screen or a printer. But to the computer, what you type and when you move the mouse and whatever you bring from the Internet or hard drive or USB stick – it all looks another part of an incredibly long series of holes in a gigantic punched card, and what you see on the screen is really just a series of new holes the computer punched as a result of what you fed it so far.

Inbal: Is this what you look at in the black screen with the green letters? The punched card inside the computer?

Yaniv: Uhm, not usually. I don’t need to. If a neurologist gets a head ache, she doesn’t undergo CT, she takes a pill. But even if I hardly ever look into whatever modern computers use to mimic their huge punched cards, I never forget this is how they are really built, and that all the wonderful things a computer can do are really performed by a giant abacus or music box with a series of holes in a long card instead of a cylinder.

Inbal: And that’s what you write about?

Yaniv: No, not yet. See, different computers, like my netbook or your iPhone react differently to cards punched in the same manner. One wouldn’t work with the other, because their internal machinery is different. You can think of it a bit like two music boxes with differently toothed combs. They’d play different music given the same pin cylinder, or, rather, one would play music and the other is likely to make icky noise, not unlike the noise you get when you tune the TV to the wrong channel.

Inbal: I hate that noise. A music box would never make that noise.

Yaniv: If it spun very quickly and had many many different teeth on the comb and had a bad cylinder inserted to it, then it would sound quite like the TV noise. The point is that different computers have different teeth combs and can play different cylinders. Python is a very special such cylinder. Python is a bit like a music box inside a music box. Python, the ‘inner’ music box affects the behaviour of the ‘outer’ music box, which is the computer. Silence. That didn’t go well. Look, you like the different apps I put on your iPhone, right?

Inbal: (frown) Mostly. Ugh. Come on, it was a farting app, I’m just a guy, how was I possibly supposed to resist it?!

Yaniv: Well, imagine an app that displays a music box on the screen and lets you tap on the pin cylinder to make your on music and play it from the iPhone. A music box inside the iPhone. That’s very imaginable, right?

Inbal: Uhm, yeah… that’s a nice idea.

Yaniv: Well, think about the pin cylinder of the music box in that app. If I would write such a program both for the iPhone and for the netbook, it makes sense that I could make it play the same music for the same pin cylinder, even tough it’s running on different computers. Right?

Inbal: Hmm-hmm. Yawn. Yikes, we better cut to the chase.

Yaniv: Well, as I said, and it’s sometimes even hard for me to believe but you have to take my word for it, that the iPhone just a punching machine itself, the music box app is really a punch card I put in the iPhone. So why not make a punching machine app? You could see it on the screen, and punch holes in a visual card, and drag it into the machine with your finger, and see the result coming out on the screen. A punching machine in a punching machine. I’d make a long punched card, this is the punching machine app, and after I feed it I will put in a second punched card, this is already running on the ‘inner’ punching machine, the app. OK?

Inbal: Mostly closed eyes, nod.

Yaniv: Well, so why not make a ‘first’ card for one machine, and a different ‘first’ card to another machine, such that the same ‘second’ card would work the same on both different machines. Just like I could program different music boxes that would behave the same way on different computers, just like Firefox acts the same on my Linux and on your Windows even though they’re different computers. Well, at least sort-of-ish.

Inbal: Long silence. Closed eyes. Ugh, she fell asleep just when I was really having fun. (sleepily) Only if the first card is really really long and the music box had many many teeth. Look, maybe her choice of words wasn’t so eloquent, but she did say a sentence very much to this effect almost in her sleep! I took that as proof she actually got what I was talking about so far.

Yaniv: (triumphant) Yes! Yes exactly! Well not exactly, but unbelievably close. Python is a very long punched card, and it runs on several different punching machines with many many teeth, but it makes all these machines behave the same way.

Inbal: My excitement must’ve woken her up a bit, because she frowned with closed eyes. All computers behave the same way? Isn’t that a bad thing?

Yaniv: For people like me, it can be a great thing! We can write software once, and run it on many different computers. And the second punch card is much easier to write compared to the first punched card we always reuse. Before things like Python, we had to think harder and punch cards differently for different computers. It wasted lots of time, so we couldn’t advance as rapidly. Today, we can do cool things in weeks. I know this is only very roughly true. But her breathing was getting very regular and I really wanted to finish the explanation. This is what I’m writing about. I’m explaining the structure of long long punched card which is Python, how it sets up the music box to be in just a particular way to eat the second punched card, which is a Python program, that’s easier to write and can run on any computer. That’s what’s my series of articles is all about, it lives happily ever after, the end.

Silence. I was wide awake and very happy. It’s so gratifying, to explain your work to your spouse. I think it’s even more so for those of us who have more arcane and less understood jobs. I stroked her dreadlocks (she puts all kinds of seashells and beads in them, it’s really nice) and finally fell asleep myself.

I can’t say her recollection the next day was perfect (You said something about music boxes… no? I remember it was a beautiful story!), but it was still quite a terrific experience for me. I wrote a very rough draft of this post in the morning when I woke up (the whole thing was a few weeks ago). As soon as I started writing it I realized this story is an obvious (and probably inferior) kin to Ryan Tomayko’s incredible “How I explained REST to my wife”. I dropped him a line to tell him I may write this piece, and he actually said some people don’t believe he had such a conversation with his wife.

Poor tossers! This is probably the most hard-core incident to date, but more often than not Inbal amazes me with how much she really knows about computers just from hanging around an aging geek like yours truly and hearing my occasional rants or explanations. Here is a plea to all you astrophysicists, microbiologists and mathematicians out there: explain your work, art and passion to your dancer, carpenter and education worker spouses! It’s fun (and you can blog about it, to boot)!