PEP 245: Python interfaces

[Raymond Hettinger]

Michel Pelletier wrote:

>
> >    d) What is also missing from Python is
> >       an "auto-delegation".  An analysis
> >       of how this fits in or doesn't fit
> >       in would be helpful.
>
> What is that?  I've neve heard of auto-delegation.
>

class AutoDelegate:
    'Mixin class autodelegates to objects listed in "delegationVars"'
    def __getattr__( self, key ):
        if not self.__dict__.has_key('delegationVars'): delegationVars=[]
        for str in self.delegationVars: # if deleVars not defined, skip
            try:
                return getattr( self.__dict__[str], key )
            except KeyError:
                pass
            except AttributeError:
                pass
        raise AttributeError, key

class Movie( AutoDelegate ):        # installs the AutoDelegate mixin
    delegationVars = ['priceCode']  # list of references to delegates
    def __init__( self, title ):
        self.priceCode = RegularPrice()
        self.title = title
    def changeCode( self, aPriceObj ):
        self.priceCode = aPriceObj
    ##def getPrice( self, qty ):    # Delegated method no longer needed
    ##    self.priceCode.getPrice(qty)

class RegularPrice:
    def getPrice( self, qty ):
        return 10*qty

class DiscountedPrice:
    def getPrice( self, qty ):
        return 10 + 9 * (qty-1)

a = Movie('Rambo')
b = Movie('Aliens')
b.changeCode( DiscountedPrice() )
print a.getPrice(10), b.getPrice(12) # these calls automatically forwarded