[IronPython] Decorators on classes

Keith J. Farmer kfarmer at thuban.org
Mon Feb 4 23:16:20 CET 2008 

Python decorators may be sugar, but .NET Attributes are not -- they're metadata that there's no way to emit, currently, without using Reflection API arcana, and no way to append to a type or member that already exists.

________________________________

From: users-bounces at lists.ironpython.com on behalf of Michael Foord
Sent: Mon 2/4/2008 2:02 PM
To: Discussion of IronPython
Subject: Re: [IronPython] Decorators on classes



Dino Viehland wrote:
> FYI IP 2.0 is tracking 2.5 and we have the big pieces in place plus many small pieces (although there's more to go).  In 1.1 we had -X:Python25 which enabled selective 2.5 features and we could conceptually do the same sort of thing for 2.0 so that it includes one or two 2.6 features such as class decorators.
>
> >From there the decorators to support attributes could even be written in Python.
>
>  

Is that right - could attributes be added to an IronPython class (or
instances) at runtime using reflection? Earlier parts of this
conversation implied that this wasn't the case...

Decorators are only syntactic sugar, so the lack of class decorators
isn't an impediment...

Michael


> -----Original Message-----
> From: users-bounces at lists.ironpython.com [mailto:users-bounces at lists.ironpython.com] On Behalf Of Keith J. Farmer
> Sent: Monday, February 04, 2008 1:40 PM
> To: Discussion of IronPython; Discussion of IronPython
> Subject: Re: [IronPython] Decorators on classes
>
> Grandfathering: Giving more consideration to retaining compatibility than it deserves. :)
>
> Obviously, IronPython should prioritize compatibility with Py2.4, but for obvious reasons I limit that to seeing IP as a consumer of CPy, not the other way around.  On the other hand, IP must also be able to consume .NET, and .NET is increasingly making use of things the IP cannot yet express.  (I thought of another framework that attributes are used on -- WCF.  There are also XML serialization attributes.)
>
> To that end, I think it would be worthwhile, for the purpose of .NET attributes, to have decorators or their analogues available to IronPython in the current stage of development.  That is, I think it should be that IronPython = CPy 2.4 + .NET Attributes + other .NET-isms expressed in a Pythonic way.
>
> ________________________________
>
> From: users-bounces at lists.ironpython.com on behalf of Michael Foord
> Sent: Mon 2/4/2008 11:53 AM
> To: Discussion of IronPython
> Subject: Re: [IronPython] Decorators on classes
>
>
>
> Keith J. Farmer wrote:
>  
>> I've had no problems with not grandfathering in older APIs, and am quite happy to not grandfather in older syntax, either.
>>
>>    
>
> What do you mean by 'grandfathering' in this context?
>
> Michael
>
>  
>> I agree that IronPython would have to be able to distinguish between CLR attributes and Python decorators, inasmuch as CLR attributes are a static part of the class/member definition.  But I don't think it's an insurmountable problem, and solving it in the DLR, actually, would be very useful.
>>
>> I wouldn't expect that class authors would want to change the set of .NET attributes frequently if at all -- it doesn't match how they've been designed and how they've evolved, so creating the attributed base class once (and then having pythonic decorators coming in and out at will) seems like a reasonable approach to me.
>>
>> ________________________________
>>
>> From: users-bounces at lists.ironpython.com on behalf of Curt Hagenlocher
>> Sent: Mon 2/4/2008 11:13 AM
>> To: Discussion of IronPython
>> Subject: Re: [IronPython] Decorators on classes
>>
>>
>> Oh! I didn't realize that about 2.6.
>>
>> There is indeed a big difference between a Python runtime decorator and a .NET compile time attribute; in fact, the similarities are superficial at best.
>>
>> .NET attributes are totally static, so there's no way to modify a .NET class definition to add them.  The IronPython engine would have to recognize particular class-level Pythonic annotations and use that information to emit a new CLR class to represent the Python class.  It already emits CLR classes as needed to represent Python classes.  By "as needed", I mean that a specific CLR base class plus a specific set of CLR interfaces will uniquely determine a class to be emitted by IronPython.  This class is cached so that -- once generated -- any new Python class definition that matches this set of (CLR base class plus interfaces) will reuse the same CLR class definition.
>>
>> What you'd have to change is to put the class-level attributes onto the generated CLR class, then change caching so that the key is (CLR base class plus interfaces plus attributes).  It's definitely doable, but raises intriguing questions about "purity".  And you'd also need to consider the impact on the larger DLR.
>>
>> Method-level attributes are an entirely different matter.
>>
>> On Feb 4, 2008 10:58 AM, Michael Foord <fuzzyman at voidspace.org.uk> wrote:
>>
>>
>>       Class decorators will be in Python 2.6 - but there is a big difference
>>       between a Python runtime decorator and .NET compile time attributes. Is
>>       it possible to attach attributes at runtime using the reflection API?
>>
>>       Michael
>>       http://www.manning.com/foord
>>
>>
>>       Keith J. Farmer wrote:
>>       > Can I resurrect this forgotten soul?  http://www.codeplex.com/WorkItem/View.aspx?ProjectName=IronPython&WorkItemId=13583
>>       >
>>       > Having just finished working on LINQ to SQL, I'm convinced that future LINQ providers will be making heavy use of .NET attributes not just on properties, but on classes themselves.  Being able to express these attributes in IronPython is a tremendous bonus.  That there be one and only one way to express these is paramount.
>>       >
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