Blog Posts from April 20, 2009 (page 1 of 1)

The Lounge Survey

I just joined The Lounge advertising network so I wanted to pass along an opportunity to win a bunch of great technical books.

The Lounge is asking the readers of the blogs in their network to fill out a survey in order for them to improve how they target their advertising. It’s pretty much what you would expect from an advertising network focused on the .NET development platform: what language(s) do you use, what framework(s), what testing tool(s), etc, etc, etc. Takes like three minutes to fill out at most.

We all know that filling out surveys isn’t what most people consider “exciting” or “fun”. In order to incent you, dear Reader, to take a few minutes of your valuable time to fill out the survey, The Lounge is giving away all forty one of Manning’s “In Action” books, including IronPython In Action. Even if you don’t win, you still get 40% discount off any purchase from Manning.

So it’s up to you, a scant few minutes of your time in exchange for a chance to win enough technical books to keep you busy for months.

Introducing __clrtype__ Metaclasses

Everyone knows Anders announced at PDC08 that C# 4.0 will include new features (aka the dynamic keyword + the DLR) that makes it much easier for C# to call into dynamically typed code. What you probably don’t know is that IronPython 2.6 includes a new feature that makes it easier for IronPython code to be called by statically typed code.

While the vast majority of .NET is available to IronPython, there are certain APIs that just don’t work with dynamic code. In particular, any code that uses Reflection over an object’s CLR type metadata won’t work with IronPython. For example, while WPF supportsICustomTypeDescriptor, Silverlight only supports data binding against reflectable properties. Furthermore, any code that uses custom attributes inherently uses Reflection. For example, Darrel Hawley recently blogged a WCF host he wrote in IronPython, but he wrote the WCF service in C#. You can’t write WCF services in IronPython because WCF expects service classes to be adorned with ServiceContract and OperationContract attributes (among many others). IronPython users want access to use these APIs. Support for custom attributes is one of the most common requests we get – it’s currently the 5th highest vote getter among open issues.

In IronPython 2.6, we’re adding the ability to customize the CLR type of Python classes. This means you can add custom attributes, emit properties, whatever you want. For those of you who’ve been dreaming of implementing WCF services or databinding in Silverlight purely in IronPython, then this is the feature for you.

In a nutshell, IronPython 2.6 extends Python’s metaclass feature that lets you to customize the creation of classes. In the metaclass, you can implement an IronPython-specific method __clrtype__ which returns a custom System.Type of your own creation that IronPython will then use as the underlying CLR type of the Python class. Implementing __clrtype__ gives you the chance to implement whatever reflectable metadata you need: constructors, fields, properties, methods, events, custom attributes, nested classes, whatever.

Over a series of posts, I’ll be demonstrating this new feature and implement some common scenario requests – including Silverlight databinding and WCF services – purely in Python. Quick warning: __clrtype__ uses low level features like Python metaclasses, Reflection.Emit and DLR Binders so these posts will be deeper technically than usual. Don’t worry – this isn’t the API interface we expect everyone to use. Eventually, we want to have an easy to use API that will sit on top of the low-level __clrtype__ hook.