Torchbox Tech

Ever found yourself trying to find the view corresponding to a url in the browser by wading though a bunch of urls.py?

This gist is one of those “why didn’t I do this sooner?” time savers. It’s a management command which given a pasted url reports back the view it resolves to, along with any kwargs/args.

Django formsets allow for validation of a maximum number of forms (partly to protect against memory exhaustion attacks using forged POST requests). However, they don't allow for validation that a minimum number of forms have been filled out. I needed this on a recent project so came up with a formset that can support this, either used directly or subclassed.

This snippet is for an inline formset but could easily be tweaked to work as a plain formset.

The blog's been quiet for a while as we've been very busy. Maybe not due to hatching plots to take over the world, but this kick-off of new posts hints that that might be happening in our free time.

My brother-in-law, Tom, has been having fun building a Lego forklift robot using a Raspberry Pi to control it. I got into the action by writing some python to use a Wiimote as the controller via Bluetooth.

A base class for general communication between the Pi and a Wiimote is subclassed to get button presses translated into motor actions.

Some fun had with bit manipulation, and now it works using simple button presses it could be extended to interpreting motion from the Wiimote.

I, for one, welcome the first of our new Wiibot overlords:

I like one liners. When debugging a python application it's sometimes handy to drop into pdb by pasting in at the relevant point:

import pdb; pdb.Pdb(skip=['django.*']).set_trace()

(here stopping pdb stepping into frames that originate in a django module).

That's great for stepping though the code and being able to change running variables when you continue (i.e. affecting the rest of the request). But sometimes you want to both play around with the variables in full python code and keep the original running variables intact when you continue the request.

So, it was very pleasing to discover a feature of IPython while reading this Reddit thread that I wasn't aware of (also pleasing that I was largely aware of the other tips!): embedding an IPython shell into running code.

Pasting this in at the relevant point:

from IPython.Shell import IPShellEmbed; IPShellEmbed()()

(yes, that's a double pair of brackets) drops you into an IPython shell with all the running variables available at that point. E.g. in a Django view you can look at your view variables, the request object, settings, etc. What's more you can play with them using any python code - for example testing a bit of the view logic within the sandbox of the shell (%cpaste magic is great for copying your code to the shell). When you're done, exit the shell and you continue with the request with no change to the original variables.

Out of interest to see how useful thumbor is as a HTTP service for thumbnailing images I built a Vagrant box which runs the service based on the Ubuntu PPA. I've packaged it up so you can build your own box in one command (somewhat quicker if you already have a local precise32 base box otherwise it'll need to download it first).

Generated thumbnails are quite nice and fast. The smart face/feature detection is variable but can give a decent result without any effort on your part. The documentation could do with some polish, particularly around examples, but overall it's a great tool.

For example, using this image:

the vagrant thumbor box creates this thumbnail:

from this request:

I had a small project that needed to serve up a form on a site (Site B) that would be shown in an iframe on another site (Site A) on a different domain. As well as submitting the form Site B needed to send some resulting data back to the Site A page.

Normally cross-domain restrictions prevent JavaScript data from being shared between the sites. However, the little known (to me at the time) window.postMessage method specified in HTML5 allows "documents to communicate with each other regardless of their source domain, in a way designed to not enable cross-site scripting attacks".

Ben Alman encapsulated it in a jQuery plugin and I used this to pass data from Site B to Site A. It was also useful in providing a way for Site A to set the height of the iframe based on the variable markup in Site B, so that scroll bars are never seen.

I've got a demo illustrating:

Sending a simple text string

Sending a package of data (passed as a param string via jQuery.param())

Resizing the iframe

We've given a bit of a spring clean to our vagrant-django-base and vagrant-django-template projects, the starting point for all our new Django builds with all our favoured tools and tech baked in.

This gist is a function that returns a generator that yields (model, instance) tuples of instances related to an object.

Ooh, been a while since the last blog post, we've been busy! However, a new year, so let's start as we mean to go on.

As part of a bigger project I needed part of a form that would allow the user to drag a marker on a map, initialised by an address, e.g. postcode, in order to get the latitude and longitude of a location.

Packaging and re-packaging Vagrant boxes can introduce bloat that means the final compressed box size grows over time making it more inconvenient to store and download. A lot of the space isn't actually part of the used virtual filesystem, it's old, no longer referenced data.

For example, I started off with a box where df reported 1.7G and was 900M compressed. After updating the OS (a lot of grind on the filesystem) df reported 2.3G and after a bit of package cleaning got down to 1.8G. However, the packaged box only compressed down to 1.6G.

How to get better compression? Write a file comprised entirely of zero bits that takes up all the remaining space on the virtual filesystem, then delete it. A sequence of n zero bits compresses just a little bit better than a sequence of n pseudo-random zeros-and-ones!

dd if=/dev/zero of=/tmp/ZEROS bs=1M ; rm /tmp/ZEROS

Ignore the out-of-space error (it's meant to fill up the space), and then package the box.

A quick self-signed SSL certificate for nginx when developing locally with HTTPS (or anything else needing one):

git config --global alias.lg "log --color --graph --pretty=format:'%Cred%h%Creset -%C(yellow)%d%Creset %s %Cgreen(%cr) %C(bold blue)<%an>%Creset' --abbrev-commit --"