Generic constructors and duplication of internal Python logic

[Michele Simionato]

jjl at pobox.com (John J. Lee) wrote in message news:<87k70bzlak.fsf at pobox.com>...
> michele.simionato at poste.it (Michele Simionato) writes:
> > Have you thought of performing the checks in the __call__ method
> > of a custom metaclass?
> 
> No.  How would that help?

I had in mind something like that:

class _WithConstructorChecked(type): # helper metaclass
    def __call__(cls, *args, **kw):
        assert len(args)<=2, "%s called with more than 2 args" % cls
        assert kw.has_key("kw"), "%s needs a 'kw=' argument" % cls 
        return super(_WithConstructorChecked,cls).__call__(*args,**kw)

class WithConstructorChecked(object): # mixin class
    __metaclass__ = _WithConstructorChecked

class C(WithConstructorChecked):   
    def __init__(self, *args, **kw):
        pass

c=C(1,2,kw=3) # ok; try different signatures to get assertion errors

In this example the mixin class WithConstructorChecked ensures that C is 
called with at least two positional arguments and a keyword argument named 
'kw'. The code of class C is not touched at all, you just add 
WithConstructorChecked to the list of its bases.

Is that what you are looking for?


               Michele Simionato