Archive for Dojo

Coding with youngsters using pgzero on RPi: Part I

An eager young coder on Step 3

Some of you will know that several-times pyweek winner Daniel Pope has put together a simple framework called pgzero to sit on top of pygame. It’s Python 3 only and is especially aimed at educational use, and we spent the last London Python Dojo coming up with examples which used it.

Its main selling point is that it supplies behind the scenes some of the boilerplate code you’d otherwise have to write to get pygame up and running in a game loop. The simplest valid pgzero game is an empty .py file! From this, pgzero will produce a blank window of the default size, which will exit if you press Ctrl-Q. Adding a few “magic” constants will get you a window of a certain width & height, with a title and its own icon. After that, just define draw() and update() and add on_<event> handlers. And you have the makings of a game. There’s a built-in Actor class which essentially acts as a sprite, and easy-to-use image & sound loading. Plus some other goodies we didn’t use.

My team was responsible for the Breakout clone at the Dojo and I took it away and demonstrated it at the next evening’s Twickenham Coding evening, a friendly meetup in a room over a pub attended by teachers, code club leaders and educators. Altho’ we’d developed on my Windows laptop, I’d already planned to use the Raspberry Pi, about which more later, and I was very pleased when it ran on the Pi without a glitch at the Twickenham meetup. The message from the teachers I showed it to (mostly at the upper-Primary range) was: still too much code. But I got some useful feedback and set to work to adapt it.

My plan was to use pgzero and the Breakout game as the basis for a short series of workshops at the boys’ club I help to run in west London. But the lads aren’t into coding as such, and I knew from past experience that they could easily lose heart and get distracted if they didn’t instantaneously produce a Call of Duty lookalike. So I aimed for two things: cutting down the code complexity so there were as few concepts as possible to get across; and breaking it up into chunks which I could present one at a time, each building on the code from the previous chunk.

The result, for the impatient, is in the piece-by-piece branch. But first, there are some yaks we need to shave.

The setup


I was keen to use the Raspberry Pi for this.

  1. It’s cheap and so the club can afford to buy several of them; and one or two of the lads have their own
  2. It’s quite different from the (often literally) black box computers they’ll generally use so they don’t immediately see it as a mere entertainment source (ie they’re more likely to enter into a spirit of producing rather than only consuming)
  3. In later sessions I hope to introduce some simple electronics via the CamJam EduKit and similar things where the RPi excels
  4. It’s a Raspberry Pi! Everyone’s talking about them. Parents are keen on them. It creates a buzz around the activity which would be lacking if I’d used some other, equally capable, platform

We already had one RPi, an original Model B; someone tweeted that RS had Model B+ going at £16 each which is well within the budget I’d set for a couple more. Fortunately we have a number of monitors (VGA), which would otherwise have been beyond us to buy. The excellent PiHut also had the necessary accoutrements at affordable prices.

As everything was taking place in our club building, which isn’t networked, I used a spare laptop setup with lubuntu to run dnsmasq for a very easy local DHCP / DNS solution. The Pis were all wired to a simple 10/100 switch.

Expecting about 6 people, I’d planned to run in a classroom mode, with my laptop able to view individual screens if anyone needed help. I’d also thought of having my screen “broadcast” to theirs. I was a little surprised that there doesn’t seem to be a definitive way of doing this. iTalc2 looked promising, but needed a fair bit of setup. There are several commercial solutions, so presumably schools are able to budget for this kind of thing. In the end, I installed x11vnc on each of the RPis. But with only 4 boys in the end, and another tech-savvy leader, it was more sensible to physically wander around to help. I do plan to look into the reverse: broadcasting my screen. We don’t have any kind of projector at the moment, and in any case I’ve never found that a satisfactory solution for showing code.

The Software

The focus of the first sessions was the use of pgzero to produce the simple Breakout clone mentioned above. But first I had to bring the complexity of the code right down, taking advantage of the fact that pgzero hides away some of the messier boilerplate. These were first-time programmers and I would be throwing a lot of new concepts at them in a fairly short space of time. (Fortunately they’re all in Years 8 to 10 at school so I didn’t have to explain too much about coordinate systems and vectors).

Responding to an issue on the pgzero tracker, I’ve submitted a PR for a pure Python implementation of the pygame Rect class. The ostensible need was to allow for floating-point size and position changes. But a secondary effect, of more immediate use to me, was the possibility of adding arbitrary attributes to a Rect instance, such as colour and direction.

So the boys were working off my fork of the pgzero repo, installed via “pip install -e .” to track the development version. They were also unwittingly working off the piece-by-piece branch of my fork of the breakout code, although I was feeding it to them a piece at a time. I’d made use of the pgzero Rect to be able to add colour and direction as attributes, which did away with the awkward global variables of the original codebase. The result, though I say it myself, is about as stripped back as you can get while still actually doing something. The global blocks setup is a little clumsy, but the alternatives I tried were no better.

The Workshop

I’d set up our own Raspberry Pis and for the one lad who brought his own I did the necessary config while they were playing football outside. I persauded them all to close down Minecraft (”Look! It’s got Minecraft!”) and Mathematica (it’s got a shiny red icon, just asking to be pressed…).

I got them all to open a terminal window and cd to ~/work/breakout where I’d cloned and then started a new branch with no files. They ran IDLE3 and started a new file called “” (or “” or “” depending on the skill of the typist). I got them to run pgzrun against their new file and use Ctrl-Q to exit the resulting window. So far, so good.

From this point on, I fed them one step at a time on paper from the piece-by-piece branch. I’d considered other options: projecting the code, having them “git checkout” to the right tag, even reading it out or live-coding it myself. But this seemed the most trouble-free option and, basically, it worked.

After they’d got each update working, I highlighted a few new or repeated features. (”That’s a name; you defined that earlier”, “That’s a function; you run all that code in one go by using the function name”) etc. One of our other club leaders, Albert, is also a professional coder (PHP) and was very able to help so between us we kept the pace going despite the variations in typing ability.

At the end of the first session they’d reached step 6 out of 12.


What worked?

  • The pacing was basically about right. One of the steps had a little too much code in it, but I needed to have a change of some sort at the end of each step, and there really wasn’t anything I could do about it
  • I was very pleased the way in which the boys worked out the obvious typing bugs; they very quickly spotted when uppercase/lowercase was wrong or when they’d misspelt a name. I very rarely had to point out anything obvious.
  • Step 4 (when the bat responds to the on-mouse-move event) was the winner. Simple as it was, the boys loved the interaction. When someone got it slightly wrong (bat.centrex = y) causing the bat to move horizontally as the mouse moved vertically, they all wanted to try it. When I suggested they swap both coordinates (bat.centrey = x; bat.centrex =y) they’d have stayed there for ages.
  • I encouraged them to change things (the screen size, the colours etc.) and I was pleased that one lad, after I’d explained how names work, changed his “bat” and “ball” names to something else — altho’ this caused problems later when I was helping him debug as I couldn’t tell which object was which!
  • Although I’d had planned for a two-hour session, the 90 minutes we ended up with was about right

What didn’t work?

  • I should have brought step 4 in a little earlier, perhaps creating the bat before the ball and then letting it move
  • Several times I had to draw things up on the whiteboard to illustrate the coordinate system etc. This probably would have worked better with a screen broadcast from an IDLE session.
  • I had intended to have them try things out in IDLE, starting from a “from pgzero.builtins import *”. This didn’t really happen, although I did get them to do some experiments with the modulo operator when explaining the colour looping for the blocks.


Overall, I was very happy with the way it worked. I had four non-programmers dive in and basically enjoy themselves. I’m hoping to use the next session to finish the game and get them to play around with bells-and-whistles (make it go faster, have a bat on each side-wall etc.). After that I hope to connect up some simple LED / buzzers to react to the game.

FWIW I’d love to do a networked version where the ball disappears off one screen and appears on the next. Each person’s ball could be colour-coded and a ball will only destroy blocks of its own colour, so you have to use balls from your neighbour’s games to finish your own. Or something. I love the idea of the social aspect that would introduce, as well as the fact that someone could have two or even more balls in play on their own screen.

Raspberry Pi-themed Dojo at BAML

Yesterday Bank of America once again hosted the London Python Code Dojo, this time at their St Paul’s offices. I don’t know what the normal use is for the room we were in, but it felt like a cross between a banqueting hall and something from Khazad-dûm. Very grand and with nice AV facilities: one big screen, several smaller screens and microphones. As is customary, the hosts provided beer & pizza, but as usual with BAML, in a somewhat classier mode, with slices and plates and a variety of drinks served by staff. All very much appreciated.

They’d also done all the setup, so we had 8 or 10 Raspberry Pi rigs with screens and enough keyboards and so on to go round. The networking took some doing and ended up being a cat’s cradle of very long ethernet cables all routing — I understand — through one all-the-data-you-can-eat phone connection! Which worked fine.

Al Broomhead and Tina Zhang gave a joint lightning talk on a couple of projects they’d been involved in on a recent Hack Day. Ben Nuttall from the Raspberry Pi Foundation had been good enough to come and join us and gave an overview of some of the resources they already have to help people use the RPi and to encourage any contributions the evening might bring forth.

Slightly differently from our usual approach, we had several different projects going during the evening. The idea was to come up with something which showcased the Raspberry Pi in some way. In the end, we had: a very simple approach to a door-challenge mechanism; a on-screen 7-segment display; an automated light which reacted to the hue of a chosen image; a text-adventure interface to Minecraft (”go forward; dig 5″); a morse-code reader using a Pibrella; and a couple of things which I’ve forgotten (sorry; someone ping me a reminder…).

At the end, of course, we had a name-draw for the book donated as usual by O’Reilly. And thanks go to BAML for their continued sponsorship of the Dojo. And to the BAML guys (and I think it was all guys) in particular for all the prep and tidy-up work to get the kit in place for us.

It was nice to see new people as well as the familiar faces. If anyone who was there wants to contribute their code or ideas to the Raspberry Pi Foundation, get in touch with Ben Nuttall. You can see a few photos via the @ldnpydojo twitter account. If any more show up we’ll try to retweet them.

London Python Dojo December 2014

I feel like a sub-editor just struggling really hard to avoid punny headlines like “A Unique Experience” and “5 Unique Solutions” and so on… because last night’s challenge was to recreate the Unix “uniq” command in Python. In fact, this was chosen only after a tie in the first round of voting giving us a top 4 rather than a top 3 and a triple-tie for second place in the second round, although with one clear winner. (Otherwise I was going to have to move onto a Single Transferrable Vote round).

It was nice to see quite a few new faces, 6 or so first-timers and several who haven’t been for a while or who have been only once or twice. Plus, of course, the old familiar faces. Fry IT continue to host us generously, including ordering artisan pizzas with enormous slices. And O’Reilly came across as always with a book for the giveaway at the end.

Most teams went for a fairly straight solution, one of them managing to carry a TDD approach pretty much throughout. One team had grander ambitions for field selection and datatype conversion, but their solution ended up on two different machines with not enough time to merge properly. I was a little surprised that no-one used itertools groupby, which is pretty much advertised as a drop-in for uniq.

You can see a few photos retweeted at the @ldnpydojo account. Back again in January, probably on the second Thursday. But watch the python-uk mailing list and @ldnpydojo for announcements.

London Python Dojo: does it have a keyboard?

Those of you who don’t frequent the London Python Dojo wouldn’t appreciate the in-joke behind the laughter which greeted the selection of “20 Questions” as the challenge for last night’s Dojo. For months Nicholas Tollervey, the main Dojo organiser, has seeded the list of Dojo suggestions with the idea of implementing a simple expert system where the computer maintains a taxonomy of objects and tries to identify an object of your selection within 20 guesses by a process of elimination. It’s become a source of humour that Nicholas has to offer the same explanation every month and that the idea is never selected as the challenge du jour. Well last night it was.

And I admit that it was much more involved than I had thought it would be. I honestly thought we’d be done in 10 minutes and looking for ways to fancy up the interface or whatever. How wrong I was! The teams split between a tree-based structure with nodes holding questions & answers and yes/no pointers; and a matrix of questions which attempted to eliminate possible objects with each question. Getting the exact looping and yes/no pointing right is harder than you might expect, especially when you’re pressed for time and you have a team of five people to consider.

We were hosted, not for the first time, by Mind Candy (of Moshi Monsters fame) in their gorgeously child-friendly offices with Mind Candy local Al Broomhead as MC. And, as always, O’Reilly provided a book for the end-of-evening raffle. I’m consistently impressed by O’Reilly’s readiness to support us: I imagine it doesn’t cost them loads to give away a book a month, but they could so easily turn us down and they continue to provide, month after month.

There have been discussions, on-list and in private, about various aspects of the London Python Dojo and the organisers have various ideas in mind, but we’re not planning to do anything drastic to the current approach. (Although our setup means we can easily experiment with something if we want). Our main idea is to emphasise our status as a beginner-friendly forum where everyone can and should get a chance to code in Python even if they’re a newbie amongst experts. I’m glad to say that, in our team yesterday, everyone got a chance to code, however lightly.

Beforehand, @ntoll gave a talk about the upcoming PyConUK and particularly about the education track, encouraging people who know or are teachers and who know or are children to come along to these special aspects of the conference. I also gave a lightning talk about the socio-political challenges in Python core development — and about how people could and should contribute.

[The title of this post refers to a commonly-asked question shouted out from the audience as the various teams were demo-ing their approaches. Demonstrating necessarily involves going through as a series of “Is it …?” “What question could I ask…?” prompts and this one turned out to be a popular question, especially if the object in mind was a Robotic Catfish!]

Python meets BoA

[tl;dr photos here]

Last night’s London Python Dojo was held, for the first time, at the very spacious Canary Wharf offices of the Bank of America. They’re big users of Python and, as we were told in an brief introductory, were keen to give something back to the Community.

They certainly did it in style. Their main reception is about the same size as Ealing Common. The meet-and-greet bar area where we had Pizza on classy platters & Beer served by bar staff is not much smaller than the whole of the offices of Fry IT, our long-standing default hosts. And the area below where a few of us gathered feels like a swimming pool with a long slide-like flight of stairs leading down. The function room where the main business of the evening was transacted was spacious with large tables (and *lots* of pencils!).

The guys at BoA had really done their prep work: power strips were already in place and every possible laptop-to-screen adapter was available. (For those who haven’t done this kind of thing: there’s *always* some kind of mismatch between a screen which can only take DVI-I and a Mac user who doesn’t have the Mini-HDMI-to-DisplayPort adapter. Or whatever: I use Windows which never has these problems ;) ).

As well as the friendly intro from one of the BoA guys, we had an enthusiastic lightning talk on Bitcoin from Sam Phippen (who comes in from Winchester or Bristol for the Dojos!). With over 30 people present, we had about 15 suggestions for the evening’s challenge, including old favourites (How does 20 Questions work, Nicholas?) and new ideas, some around the theme of banking. After the usual two rounds we settled on Steganography and made use of the generous table space (and pencils) which our hosts had provided.

The results are on Github (or will be, depending on when you’re reading this) as pull requests come in and are honoured. In short, two (three?) teams went for piggybacking on image bits; two teams (including the one I was with) encoded bits in the extraneous whitespace of a text document; and the last team tried to use the Python’s indentation to carry information in some way which I couldn’t quite understand at the time. I think that every team bar the Python-indentation one had a working result[*]; ours even had unittests!

FWIW my first idea for our team was to encode the characters in Morse code (using spaces & tabs as dots & dashes). We finally settled on binary but I still think Morse would have been cooler — and we could have played the message out as a midi file for extra points!

Of course at the end we had a draw for O’Reilly’s usual generous contribution to proceedings along with an added bonus: an historical map of programming languages. Appropriately enough, the book was won by Sal who’d been the driving force behind Bank of America hosting the Dojo this month.

Next month we’ll probably delay by a week to come in after Europython. Not sure where we’ll be yet, but follow @ldnpydojo or look out on python-uk.

And, of course, big thanks to Bank of America for being our hosts this time round.


[*] And they may have got things working after a live “Aha!” moment by Al who was demo-ing. [UPDATE: Al was actually in another team per his comment below; so many teams, so short a memory span…]