PartialInterfaces ? Re: Checking the type

Alex Martelli aleaxit at
Fri Mar 23 13:40:44 CET 2001

"Clark C. Evans" <cce at> wrote in message
news:mailman.985343914.18681.python-list at
> > # safer
> > def munge(alist):
> >     # first extract all bound-methods we'll need
> >     append = alist.append
> >     extend = alist.extend
> This is a very cool, "protocol", a partial interface.

Why, thanks.  Wish I could claim originality:-), but,
as I said, this (or a close analog thereof) is rather
standard fare in C++ templates.

> I was thinking about how this could be formalized as
> a "checker/adapter"... here is very imperfect code)
> class partial_list_checker:
>       def __init__(self, append=0,extend=0,index=0,...
>           self.check_append=append
>           self.check_extend=extend
>           self.check_index =index
>           ...
>       def check(self,alist)
>           try:
>               if self.check_append: append = alist.append
>               if self.check_extend: extend = alist.extend
>               if self.check_index:
>                  try: alist[0] = alist[0]
>                  except IndexError: pass
>               ...
>               return alist
>           except AttributeError
>              return None
> What do you think?  Kinda slow, hunh?

Rather.  Maybe slightly better:

class checker_of_methods:
    def __init__(self, *method_names_needed):
        self.method_names_needed = method_names_needed
    def check(self, anobj):
        return [ getattr(anobj, method)
            for method in self.method_names_needed ]

I'd let the AttributeError propagate up to the caller
of achecker.check if the object being checked does not
supply a needed method -- seems simples/smoothest.  And
I think returning the already-bound methods is also a
(very slight) optimization as well as a simplification.

Of course, this doesn't handle the specific check for
indexability (alist[0]=alist[0]), though I guess that
could be added as a special case (maybe in a derived
class of checker_of_methods specialized for sequence

Anyway, usage here might be, e.g.:
    append, extend = checker_of_methods(
        'append', 'extend').check(alist)


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