Context

I was asked do a presentation about developing Silverlight applications to the Melbourne Python User-group. Hell yeah, I love participating in user groups but I obviously needed some content. Unfortunately I didn't know this local user-group existed, so I've haven't been and have little idea of number of people or the type of stuff they do. This has made preparing relevant content a little challenging, but hopefully I can find the right mix.

Given it's a Python user-group I felt I might look like a bit of dick rolling with Windows and doing a demo with an environment and .NET tools that some part of the group couldn't/don't use. Besides I've always felt the Python language support and being able to build on any platform is a very compelling reason for non-.NET developers get into building Silverlight applications.

I don't yet have a Mac (I'm really starting to want one) so my other option was to do the demo on Linux. This presented some difficulties and some opportunities. Firstly I'm still a bit of Linux noob and, although I'm wrapped to have learned a lot in the last couple of weeks there has been some pain and some compromise.

Also Microsoft don't have a Silverlight browser plug-in for Linux, that support is provided by Moonlight. Moonlight is currently at version 2 and although it supports some of the features of Silverlight 3 it doesn't have the key version 3 features which have really improved the Python development environment (namely IronPython 2.6 support) and application deployment (namely allowing static assemblies that add dynamic languages support to be downloaded independently of the application and therefore cached in the browser).

Plugin and development support on both Mac and Windows is much better, but I'm happy here sacrificing some of the newer features for inclusivity (and apparently inventing words). We're targeting Silverlight 2/Moonlight 2 and effectively using no development tools besides a couple of Python scripts which I'll get onto.

Dynamic Language Silverlight Applications

Silverlight applications are packaged into binary files with an "xap" extension. It turns out these packages are just standard zip files containing all the "stuff" needed by the application. The minimum "stuff" needed in dynamic language application is;

The package can contain more Python scripts, binary assemblies (which can be imported as modules) and XAML files (which are declarative xml type format typically used for describing user interfaces). Resources such as images and video can also be included in the package but it usually make more sense for the application to download them independently when they are required.

Serving the application on a webpage is also quite straight forward. An object tag in the html has attributes to tell the browser the URL to download the application and that the Silverlight/Moonlight plugin should be used to run the application.

Helper Scripts and Templates

With a couple of templates and Python scripts we can get started developing Silverlight applications in any environment with that has Python and a browser with a Silverlight/Moonlight plug-in.

xap.py builds xaps by adding all the files from a source code directory and an assemblies directory into xap file and writing it to disk.

http_server.py is the simplest python server which just serves all the files in the current directory on a specified port.

index.html contains the object tag referencing sample.xap and a tiny bit of Javascript to catch, format and display critical error messages that may be caused by the application.

app.manifest is a template with everything needed to describe a typical Python Silverlight application. The template should be fine as-is unless additional assemblies are required or you want to change the script entry point. The template is surprisingly small and undaunting.

app.py is the script entry point of the application.

assemblies a folder with all the additional assemblies need by the Silverlight runtime to run Python scripts.

Packaging and Running an Application

So you've skipped the previous three sections and jumped straight here. I don't blame you. Here's how to get going with the script and templates described above.

Download this zip containing the assemblies, scripts and templates and unzip it someone, anywhere.

From the terminal/console/command-line go the the place you unzipped it to and run the following command to build an xap.

python xap.py samples.xap assemblies source

A sample.xap should be generated in the same directory.

To fire-up the basic web server just run

python http_server.py

Then just fire up a browsers and go to http://localhost:8000/ and you should see the sample Silverlight application running.

That's it. Now you can edit app.py to add functionality and repeat.

Going further

Clearly this a very bare bones approach to development which has very limited development and debugging tools (and a server that can only serve files). There is more tooling out there, but surprisingly litte for Python developers.

MonoDevelop 2.2, the Moonlight SKD and Python bindings may provide some sort of IDE support but it's only just been released, it is not easy to get up and running and I not sure if it even supports packaging or debugging of dynamic language Silverlight applications.

You might think Visual Studio 2008/2010 would provide significant additional support, but as far as I'm aware only the Windows Debugger which is free in the Windows SDK is much help.

Chiron is a .NET tool provided in the Microsoft Dynamic Language SDK which can package and server XAP's much like I've described above. I found I could compile it on Mono, but couldn't get it serving files. I don't think your missing much here.

XamlPad is my favourite free XAML editor, but it doesn't work on Linux. Blend and Expression Studio are pay to play Microsoft tools for designing and developing XAML. Maybe someone should write a good one in Silverlight.

IronPython can be downloaded and run on Mono.

Some Thoughts

It's easy to get started writing and deploying Silverlight applications in Python on any platform but I think it's fair to say the tooling and support is just not there yet. On any platform.

The Silverlight 2/Moonlight implementations of Python are still missing large parts of the Python standard library, which will come as a bit of a shock to Python developers and makes it difficult to import modules that depend on it. Almost the entire Silverlight 2 Base Class Library is accessible through Python but although the Python/.NET integratation is very impressive, working with the BCL does seem to limit the expressivness of the code.

Silverlight 3 appears to have some additional built-in support for interacting with the application at runtime through a Python REPL console, it supports IronPython 2.6 (which implements a lot more of the Python standard library) and has additional deployment improvements discussed above.

Finally

I hope to do another post before the meeting with some examples I'll be showing, but as always I won't promise anything (in case I get distracted writing a Socket Server in Erlang I've been thinking about). In the mean time I do have some examples on PythonSilverScripting that can be run from the browser.

I'd love to know the tools other people are using to develop Python/Silverlight and thoughts people have of it. If your in Melbourne and your at all interested you should come down to the meeting.

Update 26/01/10: Updated scripts and templates to work in Windows

Here is a zip with the bits and pieces.

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