[Cython] Visitor patterns and Python 2.4

[Dag Sverre Seljebotn]

> We ship our own Python (2.5.2) with Sage, but I would rather stick  
> with 2.3 compatibility. You might not consider it "important" but my  
> computer (OS X 10.4, yet I know 10.5 is out but I haven't found the  
> time to upgrade yet) has 2.3 by default, as did OS X 10.3.
>   
It is certainly an argument I didn't consider.

Still... Python 2.4 was released 3 1/2 years ago, and I think Cython 
users are going to be power users who will know how to upgrade their Python.

> I very much dislike using __class__.__name__ to decide what functions  
> to call, but it doesn't look like you need decorators to get around  
> this, do you?
>   
Not having decorators makes the overhead of not using the function name 
much greater. I.e. compare:

class MyVisitor(VisitorTransform):
    @pre(cls=FuncDefNode)
    def create_function_scope(self, node):
        ...

(or similar) with something like (changing the match function slightly)

class MyVisitor(VisitorTransform):
    def create_function_scope(self, node):
        ...
    create_function_scope = pre(cls=FuncDefNode, create_function_scope)


The latter one is so heavy to write that I'd rather want to simply do

class MyVisitor(VisitorTransform):
    def pre_FuncDefNode(self, node):
         ....

however I don't think this is as nice. Yes, I know, it is the standard 
visitor pattern, but I think that pattern is the way it is because it is 
made for less powerful languages than Python; using a real class 
references seems better than having (using __name__ or using a 
convention or in some way add extra, additional information to care about).

Also using decorator opens the way for other types of matches (though I 
won't propose a full XPath match again, perhaps simple "attr" matching, 
i.e. to which attribute and parent does the node belong to:

@pre(parent_cls=FuncDefNode, attr="body")
...

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Dag Sverre