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Unofficial Python Module of the Week #0: argparse

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[EDIT: This series has been renamed so as not to conflict with Doug Hellman’s excellent Python Module of the Week series. See UPMotW #1 for details]

I’m going to try something different today. This is going to be the first in a series (hopefully once weekly) of Python Modules of the Week. This has probably been done to death elsewhere, but I’m using it as an opportunity to learn some more Python and to cement some of these modules in my mind.

Argparse is a Python module for processing command line arguments. It’s new in version 2.7 and is designed to replace the Optparse module. I’m not sure what the rationale for the change was as the two modules look very similar to me. However, it suffices to say that you should use argparse in all new code.

The main class of argparse is called ArgumentParser, it allows you to add arguments to your program and parse the provided arguments at runtime:

You can also add meta-data such as help and descriptions to your arguments via keyword arguments to many of the methods. Take a look at the module documentation for examples. Once nice thing is that when you provide this data, argparse automatically generates the -h and –help flags and populates them with nicely formatted help output using the info you provide.

The argument parsing provided by argparse allows you to create some very complex command line interfaces to your scripts. I especially like the sub-commands framework, which allows each sub-command to exist as its own separate entity with its own options and help information. To create an argument parser with a sub-command is just a few lines:

Of course,each sub-command is an ArgumentParser in its own right, so you can add all the options that you normally would, including command line flags, positional arguments and help information. Also each sub-command will be listed in the help and will also have its own help page which can be accessed via:

All in all, this is a very useful Python module to know about. Its ease of use and the number of things it ‘just takes care of’ make it a pleasure to use. Hopefully it’ll have you writing nice command line interfaces to all those hacked together scripts that are currently controlled by commenting bits of them in and out!

Automating routine tasks…

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I’ve been thinking more recently about automating routine computing tasks. Using Free Software this is ridiculously easy, usually a combination of Python and Cron does the trick. More difficult I have found is actually working out what to automate. It’s very easy to get into the habit of doing something and then not realise that you do it every day, or every hour when in front of the computer.

One of my major successes in this area has been in terms of feed reading. A while ago I setup rss2email on my server to check my feeds and email the stories to me. Previous to this I actually found it really hard to ever get around to reading news feeds. I think this was mainly because checking news feeds suffers from the ‘extra inbox problem’, its just more stuff to check. Having news stories emailed to me fixes this and I can also read them easily on my phone. Because I use IMAP email, my read/unread status is also synchronised on all my computers.

Now I’m kind of at a brick wall. I don’t know what else I can set up to reduce the number of routine tasks I do. The purpose of this post is to crowd source that problem. I want everyone reading this to get back to me with the things they’ve set up to help them. You can get in touch either through the comments on this post, or via identi.ca. If I get enough responses I’ll write a follow up post detailing some of the best ideas.

The Long Awaited Kiwi Pycon Round-Up

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OK, so this isn’t going to be as large a round up as I envisaged. Principally because its been so long since the actual event and also because only one talk video is currently available – I guess all that video editing is a big job! Also, I haven’t really had the time to investigate much of the stuff I learned about over that weekend as my current Python programming is limited to the odd script here and there (most of my programming is sadly limited to C++ and Java for reasons out of my control).

Anyway, it just so happens that the video that is up is from probably the best talk of the weekend, Anthony Baxter’s ‘Pythonic APIs’. This talk made me laugh from start to finish and taught me some things I didn’t know, watch the video below if you’re interested (brought to you by the magic of HTML5!).

As far as the other videos go, I’ve subscribed to the Kiwi Pycon Blip.tv RSS feed so I’ll be able to share them as they come along.

All that remains is to talk about my overall impressions of the conference, before they completely fade from memory! Basically I really enjoyed myself, everything was very well organised and the venue was excellent. There seemed to be some rumbling that Waitangi was a bit far away from everywhere and that the conference should have been in a city. For me the location was actually a bonus as it meant I had a good excuse to take the following day off and have a bit of a holiday!

So that’s about it, thanks to Danny, Guy, Tim and everyone else who organised. I had a great time and I even won a prize. I’m just sorry this post has taken me so long to get around to writing. Maybe I’ll be quicker off the mark next year!

Calculating Alcohol By Volume in Python on Android

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Wow, I just managed to combine three of my favourite things in a single title! Recently, I’ve been getting further into home brewing, with a book I received as a Christmas present (Home Brewed Beers and Stouts, by C.J.J. Berry). Since I’d never actually measured the Alcohol By Volume (ABV) of my beer I decided to write some Python code to automate the process. The simple modules I came up with work from the command line and also on Android phones via SL4A, which makes them very useful when doing quick measurements.

Measuring ABV involves taking Specific Gravity (SG) measurements at the start and end of fermentation, adjusting them for temperature and pushing them into a simple formula. The SG measurements are taken with a Hydrometer, which is supposed to read at 20C (hence the need to adjust for other temperatures).

The temperature adjustment is done via a simple table (which I took from the book). In the script I used Linear Interpolation, to adapt it for values which weren’t available in the table. Initially I used the numpy.interp() function. However, I found that numpy isn’t present in SL4A and that installing it would be a pain since it is a C module which would need cross compiling for Android.

I therefore wrote my own interp() function, which keeps the same interface:

I split up the code into two separate modules, sg.py (which just does specific gravity adjustment) and abv.py (which does ABV calualation). Splitting the code up enables me to take SG measurements and adjust based on temperature, without doing a full ABV measurement. In sg.py the reverse adjusted SG is what you would need to read from the hydrometer at the specified temperature, in order to achieve the adjusted reading you specified.

Since the calculations are fairly trivial the main bulk of the code is in the user interfaces. I implemented a simple command line interface which either takes arguments from the command line, or prompts the user via the python input() function. I also use the SL4A API to implement a simple UI for Android, basically the user is prompted for each quantity by a dialog box.

Anyway, there’s not much else to say, except that I’ve put the code up on Gitorious for anyone who wants it (it’s licenced AGPL). The code is in a git repository for useful Python scripts I’ve written, right now it’s the only thing there, but I’m going to track down some of the scripts I’ve written over the years and add them too – I might even get a few blog posts out of some of them!

On Language/Platform Restrictions…

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Editorial Note: I know I said I was going to have a Kiwi Pycon Roundup, but I’ve been really busy recently. I’m now going to wait until the conference video/audio is posted then I can give a proper roundup and post people to the talks I found particularly interesting.

OK, today I’m having a rant. This isn’t going to be an ill conceived rant about a company, like my last one. This one is about something of a general feeling that I’ve been getting recently. I want to talk about what I call Language/Platform restriction (I’m open to a better name). This can best be described by taking an example, the best of which is Android.

So, you want to develop an Android application, but you’ve got no experience in this area. So what do you do? You head over to the Android Developer site and read some of the tutorials and documentation. To your dismay you find all the code is in Java, but you’re a Python programmer. There doesn’t seem to be any way to use your favourite language on Android, despite everyone claiming that Android is an open platform.

Granted, this example is a little contrived. The chances of any developer not knowing that Android is a Java only platform are pretty small. This isn’t even my own experience as I’m pretty good with both Java and Python (though Python is my favourite). I guess my point is that developers have preferences over which language they like to use and that these preferences seem to be being largely ignored.

Now, Android does have a working Python implementation as part of the SL4A project, but this is very much a scripting environment not an application development environment. There is no native access to the Android APIs that are required to develop a fully fledged application. Also, in my opinion its usefulness as a scripting environment is limited by the lack of a Free Software cron equivalent, to run scripts automatically.

Lest you think I’m picking on Android, this problem isn’t specific to that platform. It’s also present on The Web. “Oh No!”, I hear you cry, “The Web is an incredibly open platform, you can use any language you want!”. In some ways this is true, on the server side, yes you can pretty much use any language you want (I’ve never seen any web apps written in assembly, but I’m sure it’s possible). The client side is another matter though.

If we restrict ourselves the technologies which make up “The Open Web” (basically ignoring Flash and Silverlight), we really only have one option: Javascript. Now, Javascript has historically been a complete mess. Granted it’s getting a lot better with HTML5, but it still kinda sucks. It doesn’t have any proper OO (please don’t offer up that “a function is an object” crap, as a sorry excuse). It’s still pretty low level, certainly compared to Python and it seems to be “the language which syntax forgot”.

This worries me. It actually worries me more than the Android situation. The reason for this is that I think The Web is going to be the main application delivery platform going forward and that Free Software needs to seize on this to create a suite of Free cloud applications, just as we have for the desktop. However, the lack of a choice of languages puts me (for one) off.

There have been efforts to port Python to the browser, specifically Pyjamas and Skulpt. There are problems with both these implementations. Pyjamas is too tied to its own application framework (which come from Google Web Toolkit) and it basically ends up bringing desktop like application programming to The Web. Maybe this isn’t a bad thing, but it seems to miss the full potential of The Web. Really what we need is something that takes advantage of all The Web’s APIs, but without having to use Javscript. Skulpt, although initially promising, does the compilation from Python to Javascript on the client side, thus multiplying the overhead in doing so by the number of clients (granted its distributed, but it still seems wasteful when this only needs doing once).

I should also take a minute to mention Google’s Native Client plugin. Although its not really part of the standard Open Web technology suite, it is trying to expand beyond what Flash and Silverlight offer. Basically it’s a way to run native x86 code inside a web application and expose methods through a Javascript API. You can potentially run anything in there. There are some continuing efforts to port Python to this platform, but I think this is the wrong way to go too. It’s not really intended to be a full runtime, its more like a sandboxed version of ctypes for Javascript.

In both these situations (Android and The Web) the best option is for native support for other languages to be added directly to the underlying virtual machine. However, this doesn’t look like it’s going to happen any time soon, so we might need some other solutions. Pyjamas and Skulpt have demonstrated that it’s certainly possible to compile Python to Javascript so I think that’s a low hanging target. I haven’t seen anything more promising than SL4A for Android yet, and porting dynamic languages to run on Dalvik strikes me as much harder (given the static nature of Java).

Throughout this post I’ve kind of focussed on getting Python to run on other platforms, but what I’ve said pretty much goes for Perl, PHP or any other language. If anyone has any thoughts on this topic, or details of other projects I should check out, please get in touch. My main fear is that we’re going to end up with a situation where there is not choice of programming languages for any given platform and that some of the best languages will be left out in the cold.