Thoughts filed in: Programming

13
Aug

Using Python's namedtuple for mock objects in tests

I have become quite a fan of Python's built-in namedtuple collection lately. As others have already written, despite having been available in Python 2.x and 3.x for a long time now, namedtuple continues to be under-appreciated and under-utilised by many programmers.

# The ol'fashioned tuple way
fruits = [
    ('banana', 'medium', 'yellow'),
    ('watermelon', 'large', 'pink')]

for fruit in fruits:
    print('A {0} is coloured {1} and is {2} sized'.format(
        fruit[0], fruit[2], fruit[1]))

# The nicer namedtuple way
from collections import namedtuple

Fruit = namedtuple('Fruit', 'name size colour')

fruits = [
    Fruit(name='banana', size='medium', colour='yellow'),
    Fruit(name='watermelon', size='large', colour='pink')]

for fruit in fruits:
    print('A {0} is coloured {1} and is {2} sized'.format(
        fruit.name, fruit.colour, fruit.size))

namedtuples can be used in a few obvious situations in Python. I'd like to present a new and less obvious situation, that I haven't seen any examples of elsewhere: using a namedtuple instead of MagicMock or flexmock, for mocking objects in unit tests.

30
Jun

Splitting a Python codebase into dependencies for fun and profit

When the Python codebase for a project (let's call the project LasagnaFest) starts getting big, and when you feel the urge to re-use a chunk of code (let's call that chunk foodutils) in multiple places, there are a variety of steps at your disposal. The most obvious step is to move that foodutils code into its own file (thus making it a Python module), and to then import that module wherever else you want in the codebase.

Most of the time, doing that is enough. The Python module importing system is powerful, yet simple and elegant.

But… what happens a few months down the track, when you're working on two new codebases (let's call them TortelliniFest and GnocchiFest – perhaps they're for new clients too), that could also benefit from re-using foodutils from your old project? What happens when you make some changes to foodutils, for the new projects, but those changes would break compatibility with the old LasagnaFest codebase?

What happens when you want to give a super-charged boost to your open source karma, by contributing foodutils to the public domain, but separated from the cruft that ties it to LasagnaFest and Co? And what do you do with secretfoodutils, which for licensing reasons (it contains super-yummy but super-secret sauce) can't be made public, but which should ideally also be separated from the LasagnaFest codebase for easier re-use?

Some bits of Python need to be locked up securely as private dependencies.

Some bits of Python need to be locked up securely as private dependencies.

Image source: Hoedspruit Endangered Species Centre.

Or – not to be forgotten – what happens when, on one abysmally rainy day, you take a step back and audit the LasagnaFest codebase, and realise that it's got no less than 38 different *utils chunks of code strewn around the place, and you ponder whether surely keeping all those utils within the LasagnaFest codebase is really the best way forward?

Moving foodutils to its own module file was a great first step; but it's clear that in this case, a more drastic measure is needed. In this case, it's time to split off foodutils into a separate, independent codebase, and to make it an external dependency of the LasagnaFest project, rather than an internal component of it.

This article is an introduction to the how and the why of cutting up parts of a Python codebase into dependencies. I've just explained a fair bit of the why. As for the how: in a nutshell, pip (for installing dependencies), the public PyPI repo (for hosting open-sourced dependencies), and a private PyPI repo (for hosting proprietary dependencies). Read on for more details.

22
Jun

Generating a Postgres DB dump of a filtered relational set

PostgreSQL is my favourite RDBMS, and it's the fave of many others too. And rightly so: it's a good database! Nevertheless, nobody's perfect.

When it comes to exporting Postgres data (as SQL INSERT statements, at least), the tool of choice is the standard pg_dump utility. Good ol' pg_dump is rock solid but, unfortunately, it doesn't allow for any row-level filtering. Turns out that, for a recent project of mine, a filtered SQL dump is exactly what the client ordered.

On account of this shortcoming, I spent some time whipping up a lil' Python script to take care of this functionality. I've converted the original code (written for a client-specific data set) to a more generic example script, which I've put up on GitHub under the name "PG Dump Filtered". If you're just after the code, then feel free to head over to the repo without further ado. If you'd like to stick around for the tour, then read on.

26
Aug

First experiences developing a single-page JS-driven web app

For the past few months, my main dev project has been a custom tool that imports metric data from a variety of sources (via APIs), and that generates reports showing that data in numerous graphical and tabular formats. The app is private (and is still in alpha), so I'm afraid I can't go into more detail than that at this time.

I decided (and I was encouraged by stakeholders) to build the tool as a single-page application, i.e. as a web app where almost all of the front-end is powered by JavaScript, and where the page is redrawn via AJAX calls and client-side templates. This was my first experience developing such an app; as such, I'd like to reflect on the choices I made, and on my understanding of the technology as it stands now.

11
Aug

Mixing GData auth with Google Discovery API queries

For those of you who have some experience working with Google's APIs, you may be aware of the fact that they fall into two categories: the Google Data APIs, which is mainly for older services; and the discovery-based APIs, which is mainly for newer services.

There has been considerable confusion regarding the difference between the two APIs. I'm no expert, and I admit that I too have fallen victim to the confusion at times. Both systems now require the use of OAuth2 for authentication (it's no longer possible to access any Google APIs without Oauth2). However, each of Google's APIs only falls into one of the two camps; and once authentication is complete, you must use the correct library (either GData or Discovery, for your chosen programming language) in order to actually perform API requests. So, all that really matters, is that for each API that you plan to use, you're crystal clear on which type of API it is, and you use the correct corresponding library.

The GData Python library has a very handy mechanism for exporting an authorised access token as a blob (i.e. a serialised string), and for later re-importing the blob back as a programmatic access token. I made extensive use of this when I recently worked with the Google Analytics API, which is GData-based. I couldn't find any similar functionality in the Discovery API Python library; and I wanted to interact similarly with the YouTube Data API, which is discovery-based. What to do?

20
May

Database-free content tagging with files and glob

Tagging data (e.g. in a blog) is many-to-many data. Each content item can have multiple tags. And each tag can be assigned to multiple content items. Many-to-many data needs to be stored in a database. Preferably a relational database (e.g. MySQL, PostgreSQL), otherwise an alternative data store (e.g. something document-oriented like MongoDB / CouchDB). Right?

If you're not insane, then yes, that's right! However, for a recent little personal project of mine, I decided to go nuts and experiment. Check it out, this is my "mapping data" store:

Just a list of files in a directory.

Just a list of files in a directory.

And check it out, this is me querying the data store:

Show me all posts with the tag 'fun-stuff'.

Show me all posts with the tag 'fun-stuff'.

And again:

Show me all tags for the post 'rant-about-seashells'.

Show me all tags for the post 'rant-about-seashells'.

And that's all there is to it. Many-to-many tagging data stored in a list of files, with content item identifiers and tag identifiers embedded in each filename. Querying is by simple directory listing shell commands with wildcards (also known as "globbing").

Is it user-friendly to add new content? No! Does it allow the rich querying of SQL and friends? No! Is it scalable? No!

But… Is the basic querying it allows enough for my needs? Yes! Is it fast (for a store of up to several thousand records)? Yes! And do I have the luxury of not caring about user-friendliness or scalability in this instance? Yes!

10
May

Enriching user-entered HTML markup with PHP parsing

I recently found myself faced with an interesting little web dev challenge. Here's the scenario. You've got a site that's powered by a PHP CMS (in this case, Drupal). One of the pages on this site contains a number of HTML text blocks, each of which must be user-editable with a rich-text editor (in this case, TinyMCE). However, some of the HTML within these text blocks (in this case, the unordered lists) needs some fairly advanced styling – the kind that's only possible either with CSS3 (using, for example, nth-child pseudo-selectors), with JS / jQuery manipulation, or with the addition of some extra markup (for example, some first, last, and first-in-row classes on the list item elements).

Naturally, IE7+ compatibility is required – so, CSS3 selectors are out. Injecting element attributes via jQuery is a viable option, but it's an ugly approach, and it may not kick in immediately on page load. Since the users will be editing this content via WYSIWYG, we can't expect them to manually add CSS classes to the markup, or to maintain any markup that the developer provides in such a form. That leaves only one option: injecting extra attributes on the server-side.

When it comes to HTML manipulation, there are two general approaches. The first is Parsing HTML The Cthulhu Way (i.e. using Regular Expressions). However, you already have one problem to solve – do you really want two? The second is to use an HTML parser. Sadly, this problem must be solved in PHP – which, unlike some other languages, lacks an obvious tool of choice in the realm of parsers. I chose to use PHP5's built-in DOMDocument library, which (from what I can tell) is one of the most mature and widely-used PHP HTML parsers available today. Here's my code snippet.

30
Jun

Thread progress monitoring in Python

My recent hobby hack-together, my photo cleanup tool FotoJazz, required me getting my hands dirty with threads for the first time (in Python or otherwise). Threads allow you to run a task in the background, and to continue doing whatever else you want your program to do, while you wait for the (usually long-running) task to finish (that's one definition / use of threads, anyway — a much less complex one than usual, I dare say).

However, if your program hasn't got much else to do in the meantime (as was the case for me), threads are still very useful, because they allow you to report on the progress of a long-running task at the UI level, which is better than your task simply blocking execution, leaving the UI hanging, and providing no feedback.

As part of coding up FotoJazz, I developed a re-usable architecture for running batch processing tasks in a thread, and for reporting on the thread's progress in both a web-based (AJAX-based) UI, and in a shell UI. This article is a tour of what I've developed, in the hope that it helps others with their thread progress monitoring needs in Python or in other languages.