__clrtype__ Metaclasses: Simple Custom Attributes

I know it’s been a while since my last __clrtype__ post, but I was blocked on some bug fixes that shipped as part of IronPython 2.6 Beta 1. So now let’s start looking at one of the most requested IronPython features – custom attributes!

Over the course of the next three blog posts, I’m going to build out a mechanism for specifying custom attributes on the CLR type we’re generating via __clrtype__. All the various Builder classes in System.Reflection.Emit support a SetCustomAttribute method that works basically the same way. There are two overloads – the one I’m going to use takes a single CustomAttributeBuilder as a parameter.

For this first post, I’m going to focus on the basic custom attribute infrastructure, so we’re going to use the extremely simple ObsoleteAttribute. While you can pass some arguments to the constructor, for this first post I’m going to use the parameterless constructor. To keep things less confusing, I’m going back to the original version of the Product class, before I introduced CLR fields and properties. The one change I’m making is that I’m adding a list of attributes I want to add to the class.

from System import ObsoleteAttribute

class Product(object):
  __metaclass__ = ClrTypeMetaclass
  _clrnamespace = "DevHawk.IronPython.ClrTypeSeries"
  _clrclassattribs = [ObsoleteAttribute]

  # remainder of class omitted for clarity

Python list comprehensions use the same square bracket syntax as C# properties, so it kinda looks right to someone with a C# eye – though having the attribute specifications inside the class, rather than above it, is totally different. I wish I could use Python’s class decorators for custom class attributes, but class decorators run after metaclasses so unfortunately that doesn’t work. Also, I can’t leave off the “Attribute” suffix like you can in C#. If I really wanted to, I could provide a new type name in the import statement (“from System import ObsoleteAttribute as Obsolete”) but I thought spelling it out was clearer for this post.

Now that I have specified the class attributes, I can update the metaclass __clrtype__ method to set the attribute on the generated CLR class:

if hasattr(cls, '_clrclassattribs'):
      for attribtype in cls._clrclassattribs:
        ci = clr.GetClrType(attribtype).GetConstructor(())
        cab = CustomAttributeBuilder(ci, ())

I’m simply iterating over the list of _clrclassattribs (if it exists), getting the default parameterless constructor for each attribute type, creating a CustomAttributeBuilder instance from that constructor and then calling SetCustomAttribute. Of course, this is very simple because we’re not supporting any custom arguments or setting of named properties. We’ll get to that in the next post. In the mean time, you can get the full code for this post from my SkyDrive.

There is one significant issue with this custom attribute code. Attributes are typically marked with the AttributeUsage attribute that specifies a set of constraints, such as the kind of targets a given attribute can be attached to and if it can be specified multiple times. For example, the MTAThread attribute can’t be specified multiple times and it can only be attached to methods. However, those attribute constraints are validated by the compiler, not the runtime. I haven’t written any code yet to validate those constraints, so you can specify invalid combinations like multiple MTAThread attributes on a class. For now, I’m just going to leave it to the developer not to specify invalid attribute combinations. Custom attributes are passive anyway so I’m figure no one will come looking for a MTAThread attribute on a class or other such scenarios.

However, I’m interested in your opinion: When we get to actually productizing a higher-level API for __clrtype__, what kinds of attribute validation should we do, if any?