IronPython and Linq to XML Part 4: Generating XML

Now that I have my list of Rock Band songs and I can get the right Zune metadata for most of them, I just need to write out the playlist XML. This is very straight forward to do with the classes in System.Xml.Linq.

XElement’s constructor takes a name (XName to be precise) and any number of child objects. These child objects can be XML nodes (aka XObjects) or simple content objects like strings or numbers. If you pass an IEnumerable, the XElement constructor will iterate the collection and add all the items as children of the element. If you’ve had the displeasure of building an XML tree using the DOM, you’ll really appreciate XElements’s fluent interface. I was worried that Python’s significant whitespace would force me to put all the nested XElements on a single line, but luckily Python doesn’t treat whitespace inside parenthesis as significant. 

Creating collections in Python is even easier than it is in C#. Python’s supports a yield keyword which is basically the equivalent of C#’s yield return. However, Python also supports list comprehensions (known as generator expressions), which are similar to F#’s sequence expressions. These are nice because you can specify a collection in a single line, rather than having to create a separate function, which is what you have to do to use yield. I have two generator expressions: (XAttribute(key, trk[key]) for key in trk) creates a collection of XAttributes, one for every item in the trk dictionary and (GenMediaElement(song) for song in songs) which generates a collection of XElements, one for every song in the song collection.

Once I’ve finished building the playlist XML, I need to write it out to a file. Originally, I used Python’s built in open function, but the playlist file had to be UTF-8 because of band names like Mötley Crüe. Zune’s software appears to always use UTF-8. In addition to setting the encoding, I also specify to use indentation, so the resulting file is somewhat readable by humans.

The playlist works great in the Zune software, but since it’s a streaming playlist there’s no easy way to automatically download all the songs and sync them to your Zune device. I expected to be able to right click on the playlist and select “download all", but there’s no such option. Zune does have a concept called Channels where the songs from a regularly updated feed are downloaded locally and synced to the device. However, the Zune software appears to be hardcoded to only download channels from the catalog service so I couldn’t tap into that. If anyone knows how to sign up to become a Zune partner channel, please drop me a line.

Otherwise, that’s So there you have it. As usual, I’ve stuck the code up on my SkyDrive. If I can remember, I’ll try and run the script once a week and upload the new playlist to my SkyDrive as well.

IronPython and Linq to XML Part 3: Consuming Atom Feeds

Now that I have my list of Rock Band songs, I need to generate a Zune playlist. I wrote that Zune just uses the WMP playlist format, but that’s not completely true. Media elements in a Zune playlist have several attributes that appear unique to Zune.

Because of Zune Pass, Zune supports the idea of streaming playlists where the songs are downloaded on demand instead of played from the local hard drive. In order to enable this, media elements in Zune playlists can have a serviceID attribute, a GUID that uniquely identifies the song on the Zune service. We also need the song’s album and duration – the Zune software summarily removes songs that don’t include the duration.

Of course, the Rock Band song list doesn’t include the Zune song service ID. It also doesn’t include the song’s album or duration. So we need a way, given the song’s title and artist (which we do have) to get its album, duration and service ID. Luckily, the Zune service provides a way to do exactly this, albeit an undocumented way. Via Fiddler2, I learned that Zune exposes a set of Atom feed web services on catalog.zune.net that the UI uses when you search the marketplace from the Zune software. There are feeds to search by artist and by album but the one we care about is the search by track. For example, here’s the track query for Pinball Wizard by The Who.

Since these feeds are real XML, I can simply use XDocument.Load to suck down the XML. Then I look for the first Atom entry element using similar LINQ to XML techniques I wrote about last time. If there’s no Atom elements, that means that the search failed – either Zune doesn’t know about the song or it can’t find it via the Rock Band provided title and artist. Of the 461 songs on Rock Band right now, my script can find 417 of them on Zune automatically.

Of course, since the Zune data is in XML instead of HTML, finding the data I’m looking for is much easier that it was to find the Rock Band song data. Here’s the code pull the relevant information out of the Zune catalog feed that we need.

A few quick notes:

• The code above isn’t valid Python, I added a couple of carriage returns (albumArtist and albumTitle) to get it to read well on the blog without wrapping badly.
• song.search_string returns the song title and artist as a plus delimited string. i.e. pinball+wizard+the+who. However, many Rock Band songs end in a parenthetical like (Cover Version) so I automatically strip that off for the search string
• duration in the Atom feed is stored like PT3M23S, which means the song is 3:23 long. The playlist file expect the song length in milliseconds, so I use a .NET regular expression to pull out the minutes and seconds and do the conversion. It’s not exact – songs lengths usually aren’t exactly a factor of seconds, but as far as I can understand, Zune just uses that to display in the UI – it doesn’t affect playback at all.

Now I have a list of songs with all the relevant metadata, next time I’ll write it out into a Zune playlist file.

Early Christmas from Iron Languages and DLR

Tomorrow may be Thanksgiving, but the Microsoft DevDiv dynamic language teams are trying to make it feel like Christmas with three separate pre-holiday releases.

1. IronPython 2.0 RC2 
   We were really hoping to only have one release candidate, but we ended up with a couple of significant bugs that we couldn’t push off to 2.0.1. With December holidays coming soon, RC2 has a pretty small window before we declare RTM so now is the time to download the release and try your code out.
2. IronRuby 1.0 Alpha 2 
   There’s been zero blog traffic on this, just a notice on the IronRuby mailing list. As per said notice, “Notable features” include “the inclusion of iirb.bat, igem.bat, irails.bat, irake.bat”.
3. New DLR CodePlex Project T
   he DLR source has been available as part of IronPython for over a year but now they have their own home on CodePlex. Check out the Release Notes for an overview, reads some Docs and Specs or just download their initial v0.9 beta. Their v0.9 beta is synced with IPy 2.0 RC2 (and their v0.9 final will sync with IPy 2.0 RTM) but it also includes synced versions of IronRuby and ToyScript in both source and binaries. Plus, Sesh has promised “weekly code drops”. Finally, unlike IronPython and IronRuby, DLR is using the discussion section of their CodePlex site – I’m eager to see how well the new-ish discussion/mailing list integration works.

So there you go, new versions of IronPython and IronRuby plus a whole new DLR CodePlex project to boot. Enjoy.

IronPython and Linq to XML Part 2: Screen Scraping

First, I need to convert the HTML list of Rock Band songs into a machine readable format. That means doing a little screen scraping. Originally, I used Beautiful Soup but I found that UnicodeDammit got confused on names like Blue Öyster Cult and Mötley Crüe. I’m guessing it’s broken because IronPython doesn’t have non-unicode strings.

Instead, I used SgmlReader to provide an XmlReader interface over the HTML, then queried that data via Linq to XML. I used the version of SgmlReader from MindTouch since they include a compiled binary and it seems to be the only active maintained version. I wrapped it all up in a function called load that loads HTML from either disk or the network (based on the URI scheme) into an XDocument.

I call load, passing in the URL to the list of songs. The “official” Rock Band song page loads the actual content from a different page via AJAX, so I just load the actual list directly via my load function.

Once the HTML is loaded as an XDocument, I need a way to find the specific HTML nodes I was looking for. As I said earlier, XDocument uses Linq to XML – there is not other API for querying the XML tree. In the HTML, there’s a div tag with the id “content” that contains all the song rows as table row elements. I built a simple function that uses the LINQ Single method to find the tag by it’s id attribute value.

(Side note – I didn’t like the verbosity of the “a != None and a.Value == id” line of code, by XAttributes are not comparable by value. That is, I can’t write “node.Attribute(‘id’) == XAttribute(‘id’, id)”. And writing “node.Attribute(‘id’).Value == id” only works if every node has an id attribute. Not making XAttribute comparable by value seems like a strange design choice to me.)

LINQ to objects works just fine from IronPython, with a few caveats. First, IronPython doesn’t have extension methods, so you can’t chain calls together sequentially like you can in C#. So instead of collection.Where(…).Select(…), you have to write Select(Where(collection, …), …). Second, all the LINQ methods are generic, so you have to use the verbose list syntax (for example: Single[object] or Select[object,object]). Since Python doesn’t care about the generic types, I wrote a bunch of simple helper functions around the common LINQ methods that just use object as the generic type. Here are a few examples:

Once I have the content node, all the songs are in tr nodes beneath it. I wrote a function called ScrapeSong that transforms a song tr node into a Song object (which I’ll talk about in the next installment of this series). I use LINQ methods Select, OrderBy and ThenBy to provide me an enumeration of Song objects, ordered by date added (descending) than artist name.

And that’s pretty much it. Next, I’ll iterate thru the list of songs and get the details I need from Zune’s catalog web services in order to write out a playlist that the Zune software will understand.

IronPython and Linq to XML Part 1: Introduction

Shortly after I joined the VS Languages team, we had a morale event that included a Rock Band tournament. I didn’t play that day in the tournament since I had never played before, but I was hooked just the same. I got Rock Band for my birthday, Rock Band 2 shortly after it came out in September and I’m hoping to get the AC/DC Track Pack for Christmas.

There are lots of songs available for Rock Band - 461 currently available between on-disc and downloadable tracks – with more added every week. Frankly, there’s lots of music on that list that I don’t recognize. Luckily, I’m also a Zune Pass subscriber, so I can go out and download all the Rock Band tracks and listen to them on my Zune. But who has time to manually search for 461 songs? Not me. So I wrote a little Python app to download the list of Rock Band songs and save it as a Zune playlist.

I ended up use Linq to XML very heavily in this project. Zune playlists use the same XML format as Windows playlists, Zune exposes the backend music catalog via a Atom feeds and I used Chris Lovett’s SgmlReader to expose the HTML list of Rock Band songs as XML. I realize Linq to XML wasn’t on “the list”, but I had a specific need so it got bumped to the head of the line.

BTW, for those who just want the playlist, I stuck it on my Skydrive. Unfortunately, there’s no Skydrive API right now, so I can’t automate uploading the new playlist every week. If anyone has alternative suggestions or a way to programmatically upload files to SkyDrive, let me know.