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Trouble with metaclass

Michele Simionato mis6 at pitt.edu
Thu Nov 13 08:51:25 EST 2003 
Fernando Rodriguez <frr at easyjob.net> wrote in message news:<jbq6rvob5qjddbtt6vk97gsr6hk993e6j3 at 4ax.com>...
> Hi,
> 
> I'm having trouble with a metaclass suposed to check the method signature of
> its classes.
> 
> Here's the metaclass:
> 
> class MetaChecker(type):
>     def __new__(cls, name, bases, attribs):
>         for name, value in attribs.iteritems():
>             if  inspect.ismethod(value):
>                 if not isSingleArg(value):
>                     raise "%s is not a thunk method!"%name
>         return type.__new__(cls, name, bases, attribs)
>     
>  
> def isSingleArg(fn):
>   
>     args = inspect.getargspec(fn)[0]
>     if len(args) == 1:
>         return 1
>     else:
>         return None
>    
> 
> And here's a class with this metaclass:
> 
> class Preconditions (object):
>    
>     __metaclass__ = MetaChecker
> 
> 
> 
> If I define a descendant of Preprocesor with a method with 2 arguments, I
> don't get any error:
> class P (Preprocessor):
>    def f(self, x):
>       return x
> 
> 
> What am I doing wrong? O:-)

The metaclass is right and actually the names you are using (Preconditions/
Preprocessors) suggest to me that you are using the right tool for the job;
the only problem is that you must use 'inspect.isfunction' and not
'inspect.ismethod', because of the methods <=> functions mismatch I am
sure you know everything about ;) If not, please feel free to ask.

Also, I would use

   return super(mcl,MetaChecker).__new__(mcl,name,bases,attribs)

instead of type.__new__, anticipating the need for multiple inheritance
of metaclasses.


              Michele
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