PyCon UK 2013 Notes

Over the weekend, I was at PyCon UK 2013, which was fantastic! I took notes of some of the talks I went to, and the links to my Evernote notes are below:

• Tim Golden: Ten Things Coming in Python 3.4
• David Winterbottom: A Smorgasbord of Testing
• The Test People: Breaking Things for Money
• Rob Collins: Test-Driven Madness

For more PyCon UK follow-up goodness, check out the conference wiki.

Fixing nose.tools pylint errors

What is nose.tools?

If you've engaged in any test-driven Python development, you've probably considered using nose as your test runner. One of my favourite parts of nose (among its many excellent parts) is the nose.tools module. This not only contains a number of helpful tools (documented here), but also all of the assertion methods on unittest.TestCase converted to PEP-8 compatible functions (e.g. self.assertEqual becomes assert_equal).

These assertion functions are useful for a few reasons: readability, line length, PEP-8 compliance. Most importantly, though, is the fact that nose allows you to write test functions and test classes which don't subclass unittest.TestCase. The only way to access these assertions, therefore, is through nose.tools.

The assertion functions in nose.tools are generated at runtime from unittest.TestCase and put in to the nose.tools namespace. This has a couple of consequences: firstly, any new assertions in the standard library will immediately appear in nose.tools (a word of warning: as nose doesn't implement the assertions, you need to pay attention to stdlib unittest assertion changes). Secondly, this means that they don't appear anywhere in the source code if you go looking for them. This will become relevant shortly.

What is pylint?

If you've engaged in any team Python development, you've probably considered pylint to help you maintain code quality. pylint uses static analysis to find problems in your code. Static analysis just means that it looks at your code as text, rather than executing it (which would be dynamic analysis). The problems it finds range from bad syntax to poor style; it can even work out when you've tried to import a non-existent symbol from another module.

What's the problem?

This last feature is really useful most of the time; it has saved me from pushing broken code innumerable times. However, it does cause one major headache: because the nose.tools source code doesn't contain the PEP-8 assertion functions (remember, they're dynamically generated!), pylint's static analysis can't find them; every single time you import one of them you get a pylint error (E0611, to be precise). If you're using nose as part of your continuous integration to check your pylint errors, this can be a major source of pain. Luckily, pylint has a solution: disabling checks.

Fixing with disabled checks

pylint allows you to disable checks in a couple of ways: you can disable them globally (i.e. none of these warnings should ever be displayed), or based on scope (i.e. this module, class, function or line should never have warnings displayed). So, for example, an import line could look like:

from nose.tools import assert_equal  # pylint: disable=E0611

Problem solved! Your pylint run will no longer flag this line up as problematic. There are a few issues with disabling checks, though. Firstly, you have to remember to do it. Secondly, it crufts up the screen when you're trying to read, and gives you more information that you need to process. Finally, and often most crucially, you can accidentally change the scope of your disabling. Consider the following lines at the top of a module:

# pylint: disable=E0611
from nose.tools import assert_equal, assert_items_equal, assert_greater

This does solves the warning (and avoids violating PEP-8's line limit), but it has an unintentional consequence: E0611 is now disabled for the entire file. pylint will no longer catch this obvious error!

# pylint: disable=E0611
from nose.tools import assert_equal, assert_items_equal, assert_greater
from mcok import Mock

Fixing with a pylint plugin

Luckily, there is a better way of solving this problem than disabling individual checks everywhere. pylint provides a plugin interface so that people can implement their own checks and plugins. Using this, we can also modify the way that the static analysis looks at the nose.tools module. In this pylint plugin, I have implemented a function which transforms the abstract syntax tree which pylint uses when looking at nose.tools to inject all of the functions which are generated at runtime:

from nose import tools

function_template = """
def {}(*args, **kwargs):
    pass
"""

def nose_transform(module):
    funcs = ''.join(function_template.format(func_name)
                    for func_name in tools.__all__)
    fake = ASTNGBuilder(MANAGER).string_build(funcs)

    for func_name in tools.__all__:
        if func_name not in module.locals:
            module.locals[func_name] = fake[func_name]

Just follow the installation instructions in the gist, and all of your pylint nose.tools problems will disappear!

BrightBox munin plugin

We're signed up with EE for broadband, and they provide us with a BrightBox router. I wanted to see what connection speeds we were getting over time, so I wrote a munin plugin which would graph it for me.

You can find the source, installation instructions and sample output on GitHub.

Zero-Hour Contracts: Letter to my MP

Earlier this evening, I read the Guardian article about Sports Direct's zero-hour policy for their part-time staff. After my wife pointed out that a bitchy outburst on Twitter wasn't going to change anything, I decided to write a letter to my MP, Kate Hoey, using the excellent WriteToThem service.

If this issue (or any other) winds you up as much as it does me, I would recommend using some of your self-righteousness to write a letter to your MP. How I did so is enclosed below.

Dear Kate Hoey,

You may recently have read in The Guardian about Sports Direct's policy of keeping all of their part-time employees on zero-hour contracts[1].                                                                                      
                                                                                                                                                            
I feel like zero-hour contracts are unacceptable, as they leave employees with no sick pay, holiday pay or guarantee of future earnings. This sort of security is something that I feel an employer should provide their employees with.

I would appreciate hearing your thoughts on this matter which I consider very important.

Yours sincerely

Daniel Watkins

[1] http://m.guardian.co.uk/business/2013/jul/28/sports-direct-staff-zero-hour-contracts

Sushi Go! Unboxing

A few months ago, I supported Sushi Go! on indiegogo. Sushi Go! is a set collection card game, similar in mechanic to 7 Wonders (one of my favourite games).

Today, it arrived! I haven't had a chance to play it yet (as I'm writing this at work), but here are some photos.

The Box

The box is very small, and very pretty. I'm pretty sure this is the cover design which I voted for, but I am obviously in no way biased.

Opening the Box

To give you some idea of the size, the cards are slightly smaller than standard playing cards (but big enough for my large hands to handle comfortably, I think!).

The Rulebook

The rulebook is smallish but, again very attractive!

Everything Side-by-Side

Some of the Cards

All in all, I'm a very happy camper!

Version 3 of sphinx-git Released

I've just pushed the latest release of sphinx-git to PyPI.

sphinx-git extends Sphinx, adding a directive which allows you to embed a git changelog of your documentation within the documentation itself.

sphinx-git is developed on GitHub where you can find an example project. Issues, comments or pull requests welcome!

Using inotifywait To Run Your Tests When Your Code Changes

Recently, I tweeted about my desire for a change-aware test runner:

I would like a test runner that detects when I have changed tests/code, and runs the relevant tests.
— Daniel Watkins (@Odd_Bloke) January 31, 2013

I haven't quite come up with that (though I do now have a half-finished blog post prognosticating on it), but I do have a solution which covers some of the bases: I determine what tests I want to run, and the test harness runs them whenever my code changes.

This "test harness" is actually a bash snippet which relies on inotifywait, a command-line program which blocks until it detects an inotify event on the files it is watching, and is incredibly simple:

while inotifywait -e close_write -r $CODE_DIRS --exclude=".*sw[px]"; do
    $TEST_COMMAND
done

This while loop is simple. When the inotifywait command stops blocking (and with exit code 0, which it will unless something unusual has happened to the files you're watching), we run $TEST_COMMAND. You can put whatever you want there, so you could limit the number of tests you run that way.

Now let's break that inotifywait call down. $CODE_DIRS can be any number of directories or files that you would like inotifywait to watch. -e narrows down the events that we should unblock on; we don't want to run the tests every time we open a new file. close_write triggers on (to quote the man page):

file or directory closed, after being opened in writable mode

vim triggers this when I save, so it works for me. You might also want to listen for modify (file or directory contents were written), move (file or directory moved to or from watched directory) and create (file or directory created within watched directory). Multiple -e options are comma-separated.

-r, as with many commands, tells inotifywait to watch directories recursively. This will mean that all directories under those that you specify will trigger your tests. I normally just run this command pointing at the top-level directory of my project. It's worth noting that -r applies to all directories that you pass.

Finally, --exclude takes a regular expression of files to exclude from your watch. As a vim user who hasn't configured swap files to be stored out of my tree, I want to ignore them (otherwise my tests run every time I open a file because vim writes out a swap file).

A quick disclaimer: as this uses inotify, this will only work on platforms that support it (which, I think, is only Linux). Mac users might want to examine this StackOverflow question. Windows users might find this answer useful.

UPDATE: +Murali Suriar has helpfully pointed me to kqwait on IRC, which will help out any Mac/BSD users.

Removing .pyc Files: A Coda

A few days ago, I wrote a blog post detailing a git hook that would automatically remove .pyc files when checking out a different branch. I received a variety of feedback, which I will outline here.

Firstly, +Jeff Mahoney provided a more efficient implementation of the git hook in a comment on the original post. I haven't tested this, but it might provide a more efficient implementation if you need it.

Secondly, there are a number of helpful comments on the reddit post, including a fine-tuned git/xargs command.

Finally, and most importantly, a number of people (both in the blog comments and on reddit) pointed out the PYTHONDONTWRITEBYTECODE environment variable.  Setting this to anything will mean that Python doesn't generate .pyc or .pyo files, completely circumventing the problem that I was trying to solve.

Git Tip: Get A Warning When A File Has Changed

Every project has one¹. The file. If that file changes, you need to know, because if you don't then your life is going to be very unpleasant in very short order.

At Hogarth, that file is called development.conf. It is, roughly speaking, a dump of the way our various elements and components are wired together², and as it's completely unmergable only one person can be working on it at any one time (something which we're working to fix). If you want to work on it, you need to make sure that you have the latest version in the database, so that when you dump out your new version, it includes all previous changes.

At various times, we've all been caught out by missing an update, and had to completely redo (often complex) changes just to incorporate a (often minor) previous change. One of my colleagues (Patrick), having just been caught out by this for (I think) the first time, suggested that it would be useful to get a warning when this changes. He reasoned that as we use git to manage all change to our codebase, it would be natural to write a git hook which did this for us.

We had a look together at the list of git hooks, and couldn't really see anything appropriate. post-merge seemed like what we wanted, but we couldn't work out how we could determine what had actually changed. So Patrick went off to do some real work, and I turned to the ever-reliable #git IRC channel on Freenode. ojacobson suggested a solution, which works beautifully.

To understand how this works, you'll need to know about the git reflog. +Alex Blewitt has a good introduction here. If you do one thing as a result of this article, educate yourself about the reflog. It's incredible!

The important take-away from that article is that:

git diff "HEAD@{1}"

will show you the diff between what you currently have in your tree and the commit before the last action that changed your history. Importantly, it treats merges as a single entry, so if you are immediately post merge, running the above command will show you a diff containing everything that changed in that merge, regardless of the number of commits the merge contained.

Pulling these various bits together, with some shell wizardry, gives us:

#!/bin/bash
set -eu

git diff "HEAD@{1}" --name-only | grep config/development.conf 2>&1 > /dev/null
CHANGED=$?

if [ $CHANGED ]; then
    echo
    echo -e "\e[41m!!! development.conf HAS CHANGED !!!\033[0m"
    echo
    echo "You should reload config using (something like):"
    echo "  ./manage.py loadconfig config/development.conf"
fi

We check if anything has changed, and if it has then we print out a BRIGHT RED WARNING MESSAGE. Drop that in .git/hooks/post-merge, make it executable, and you're good to go.

---

1 Well, maybe not every project. If you don't, I'm jealous.  But a lot of "enterprise" or otherwise unloved codebases will have one. And, more pertinently, ours does.^↩

2 You can tell that ours is not a Java project because this is not an XML file.^↩

Git Tip: Remove .pyc Files Automatically

N.B. I've published a follow-up to this here, which includes a way to completely avoid this problem in the first place.

Recently, I've found myself increasingly caught out by stale .pyc files in our project. When I change from our mainline branch to a story branch (or vice-versa), I often find myself with inexplicable test failures because Python is using the .pyc files for no-longer-current code.

Luckily, it's pretty easy to fix this in git, using hooks, specifically the post-checkout hook. To do that, add the following to .git/hooks/post-checkout, and make the file executable:

#!/bin/bash
find $(git rev-parse --show-cdup) -name "*.pyc" -delete

Now, every time you checkout a new branch, all the .pyc files will be cleared out of your git branch.