Data for Good ?


A short post to share the slides I used during my talk at the 4th PyData London Meetup. Organized by Ian Oszvald, co-Taarifian Florian Rathgeber, and others, its always a full house with a friendly crowd.

Given the limited time I had to put something together I think the talk was well received and triggered lots of good feedback and conversations in the pub afterwards. Hopefully I managed to ‘turn’ a few attendees ūüôā

Aside: There has been a lot happening on my end the last couple months and its been tough (but exciting!) keeping up with it all. My digital prescense has been lagging behind somewhat but slowly getting there.



PyData London: Modeling Departures from Routine in Human Mobility

BhLGtxUIUAA7U7XOne of the projects I have been involved with recently was a collaboration with the Agents, Interaction, and Complexity group at University of Southampton. The same group who we are also involved with in the Orchid project. This particular project was on Measuring and Predicting Departures from Routine in Human Mobility, building on the PhD work by James McInerney (now at Princeton) and his paper Breaking the Habit.

It is well known that humans generally follow very regular and predicable mobility patterns, both spatially and temporally. Lots of work has looked at exploiting and predicting those patterns but much less so on looking specifically at departures from those regular patterns. What can we learn from those departures from routine? How predictable are they?

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Raspberry Pi GPIO Workshop

Update: a writeup of the event by Lorraine is here.

A shot post to say that I am co-organizing a Raspberry Pi workshop with The IET Solent Branch at the University of Southampton on 25 April. For personal reasons I cannot make it myself that day but do go along if you want to learn about how to use the GPIO pins on your Pi board!

Raspberry Pi - 25 April Poster

Btw, if you can’t make it but live in the area, do checkout Recently launched and we now have a space!


Rewired State Parliament hack weekend 2012

Its 8:44 am and after 5 hours of sleep on my trusty Thermarest I feel quite refreshed, which is more than I can say about the people around me. Some have capitulated and lay scattered around the brightly lit room under their coats in front of their MacBook Air’s. Others are still in exactly the same position I left them 5 hours ago but the intensity has gone and eyes have glazed over. At least one person confirmed the geek stereotype and didn’t manage to hold his beer.

Update: Apparently Parlycloud won a special mention during the judging, thanks!

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Coursera-dl: A Coursera download script

I have blogged about in the past and as part of signing up to a number of courses I felt the need to easily download the videos, quizzes, notes, etc. locally for later use offline.

I quickly found a project on github (and there are a few) but wasn’t quite happy with the code. I cleaned it up to a relatively sensible state and it now does what I wanted it to do. The main additional features I wanted were: easily download multiple courses, support for quizzes/homeworks, and support for links to extra material (e.g, 3rd party sites, papers, etc).

Just do a “pip install coursera-dl” and then run as follows:

coursera-dl -u myusername -p mypassword -d /my/courses/ algo-2012-001 ml-2012-002

Code is in python and can be found on Github.

Some people have asked if they could donate something. If you wish you can do that here:

Donate Button

Update: if you have a feature request or want to report a bug please use the github issue system


Bridging Python and Matlab: Passing callbacks and optimization

This blog post is about a bridge I created that allows you to pass callbacks to Python functions directly to Matlab code without having to resort to things like sockets or COM.


Designing our UAVs involves a number of different codes that interact & must be coordinated to solve what is essentially a Multidisciplinary Design Optimization (MDO) problem.  One of those codes is written in Python and involves a constrained optimization problem of the following kind:

Find the minimum of a problem specified by

x, b, beq, lb, and ub are vectors, A and Aeq are matrices, c(x) and ceq(x) are functions that return vectors, and f(x) is a function that returns a scalar. f(x), c(x), and ceq(x) can be nonlinear functions.

In our case we have 10 variables (x has length 10), 28 nonlinear constraint functions c(x), 4 nonlinear equality constraint functions ceq(x) and box constraints on most variables.  The objective function f(x) and all constraint functions are pure python functions.

Thus to optimize f(x) you need an optimizer that can handle python functions, and for that there is the excellent openopt library.  This is what I did and after trying out the different NLP solvers I settled on the scipy_slsqp solver for most runs as it seemed to consistently give the best results.

However, as the optimization problem was tweaked and as we did more runs, we did run into cases where it would not solve and the aeronautical engineers I work with were not always happy with its robustness.  Also nobody had ever worked with the openopt library but everybody knew and trusted the various Matlab solvers.  Besides, us being part of the Computational Engineering Design Group, a number of powerful in-house Matlab solvers had been developed and were available to me.  Thus, the least I could do was to try to use those and see if that improved anything.

Python-Matlab bridge

So I started searching the web for tools / libraries that allowed me to bridge python/Matlab, specifically tools that supported passing callbacks between the two languages.  A promising project seemed mlabwrap but while it was great for evaluating Matlab expressions,  it does (at the time of writing) not support callbacks.

In the end I decided to see if I could scratch my own itch.  I really did not want to deal with sockets or any kind of network protocol and I also wanted the solution to be crossplatform, so no COM.  Since both Python and Matlab have well documented interfaces with C/C++ code, that seemed to be the way to go.

So I sat down with the ctypes & mex documentation and with a little help from Mathworks support was able to cobble together a simple example using fminsearch. ¬†By the way, I had never done any Python – C work before, but ctypes is awesome and a joy to use. ¬†The little trick to make it all work is that you just have to treat the function handle as an integer (it’s just a memory address after all). ¬†Then all you are really doing is passing an integer around which is straightforward.

With fminsearch working the goal was to extend to fmincon since that would give us all the basic functionality needed.  This took a bit of debugging and deciphering Matlab crash dumps but I finally got it working.  The final flow of events looks like this:

You start off with writing a matlab file that wraps the optimizer you are interested in (in this case fmincon) and exposes its signature.  This m file forwards all function handle arguments to a mex file which converts the integer to a function pointer and calls it.  This is how we get the callback to a C function.  A simple bridge library is then needed which is loaded by ctypes and converts the arguments into something the Matlab shared library file understands.

From my initial tests I see it is finding the same optima as scipy_slsqp was using openopt. Now I just need to do more extensive testing to see if performance is any more robust. After that I can start looking into the other solvers provided by Matlabs optimization toolboxes as well as our own solvers.

Source code

All the code is available on github. ¬†I hope it is useful to somebody and I’d be very happy to receive patches or improvements. ¬†I guess this may even be of use to the mlabwrap / openopt projects.


Imperial College Robotics Society Talk

I meant to post this some time ago but craziness at work and an overdose of family during the festive period intervened.

Anyway, mid December I did a talk of our UAV work at the Imperial College Robotics society.  I had met Harry Eakins, founder and current president, at the last pyconuk conference in Coventry & he invited me to give a similar talk at Imperial.

As usual, the slides are available on slideshare:

“Click here” to build your UAV

View more presentations from dgorissen

In all I found it a great experience.  I tried to give a broad overview of the UAV related activity at the university as well as focus more on our research around DECODE.  Got a good audience with some great discussion during the reception afterwards.

On arrival I got a tour of their¬†hackerspace¬†and it was great seeing people hacking and soldering away on various projects, ranging from quadcopters to a robotic dustbin. ¬†Thinking back to my university days I always regret not having gotten involved more in such things or set them up if they did not exist. ¬†There are¬†so many cool things¬†you could do and the technology & kit is very accessible. ¬†These days my tinker time is very limited but Im optimistic in that will change ūüôā