[Python-3000] Specializing the dicts in __dict__

Ian Bicking ianb at colorstudy.com
Tue Apr 18 19:10:15 CEST 2006

Steven Bethard wrote:
> Could someone put out some use-cases where it would be really helpful
> to be able to specify a different dict type for the class dict?

In SQLObject and FormEncode, you can do something like:

   class Registration(formencode.Schema):
       fname = String(not_empty=True)
       lname = String(not_empty=True)

I'd like to know the order of fname and lname internally; in SQLObject 
I'd like to know the order of the columns.  You can get away without 
knowing the order, but the result feels distinctly sloppy.  Or else you 
get stuff like:

   class Registration(SomeClearlyInferiorSchemaClass):
       fields = [String('fname', not_empty=True),
                 String('lname', not_empty=True)]

In this case the classes and objects are all cooperative, and the 
problem can be resolved by tracking the order in which these String 
instances were created, though it only allows for the ordering of like 

Another case is document generation through introspection; usually 
methods and instances were ordered in the source for some reason, but 
this ordering is lost.  You can order the methods by looking at their 
line numbers (which is tracked), but you can't order attributes, you 
can't order descriptors, and you can't interleave attributes with methods.

In the case of documentation, you can't expect the classes and objects 
to be cooperative, and you can't expect metaclasses to provide ordered 
dicts for their implementations (unless you were able to monkeypatch the 
entire system to use ordered dicts, which would be acceptable for 
documentation generation even if it slowed the system down).

In the case of documentation, AST inspection might be a better way to 
go, but it does limit the accuracy of the documentation in other ways as 
nothing dynamic done at import time can really be understood without 
actually importing the code.  So the better systems typically have to do 
a bit of both source and object inspection.

Ian Bicking  /  ianb at colorstudy.com  /  http://blog.ianbicking.org

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