Code generator and visitor pattern

[Stefan Behnel]

MRAB, 15.07.2010 21:33:
> Stefan Behnel wrote:
>> Karsten Wutzke, 15.07.2010 20:45:
>>> Well, I'm most experienced in OO, so writing OO in Python seems like
>>> the way to start with Python. The visitor pattern uses single-
>>> dispatch, that is, it determines which method to call be the type of
>>> object passed in.
>>
>> Well, then do that. Put the types into a dict and map them to the
>> functions to call, then call the function through the dict. That's a
>> pretty common way to do dispatching.
>>
>>
>>> Note, that I have an hierarchical object structure which I want to
>>> iterate over, so using OO looked natural to me. If there's a better
>>> approach, I'm all ears.
>>
>> You speak in riddles, but my guess is that your problem is that you
>> don't want to dispatch mechanism to match only exact types but also
>> subtypes. No problem, just build your dict incrementally and add new
>> types as they come in. See this file for an example:
>>
>> http://hg.cython.org/cython-devel/file/tip/Cython/Compiler/Visitor.py
>>
> Another variation: the dispatch table starts with entries for certain
> types then it adds subtypes on demand:
>
> def visit(self, obj):
> try:
>     handler = self.dispatch_table[type(obj)]
> except KeyError:
>     for disp_type, disp_func in self.dispatch_table.items():
>         if isinstance(obj, disp_type):
>             self.dispatch_table[type(obj)] = disp_func
>             handler = disp_func
>         else:
>             raise RuntimeError("Visitor does not accept object: %s" % obj)
> return handler(obj)

Well, yes, that's basically what the code behind the above link does, 
except that it follows the type hierarchy correctly to find the closest match.

Stefan