method overriding trick

David Ascher da at ski.org
Thu Nov 11 18:22:30 CET 1999


On Thu, 11 Nov 1999, Jeremy Hylton wrote:

> Overriding methods of a base class is a little clunky in Python.  You
> need to explicitly name the base class that implements the method, and
> you call its as an unbound method, passing self as the first argument.
> Example: Base.aMethod(self, arg)
> 
> I was reading about MzScheme the other day (a Scheme variant with
> object and interface support), and noticed that the MzScheme approach
> translates well to Python.  MzScheme class defintions allow a rename
> declarations that create a name in the class being defined that is
> bound to a name in a superclass.  In Python, rename is just called
> assignment. 
> 
> class Base:
>     def aMethod(self, arg):
>         return arg * 2
> 
> class Derived1(Base):
>     super_aMethod = Base.aMethod
> 
>     def aMethod(self, arg):
>         return self.super_aMethod(arg) + 1
> 
> I haven't seen this strategy for overriding mentioned before, although
> I did not search the list archives.  When I described it to Guido, he
> said, "Does that work?"  Has anyone used this trick before?
> 
> It doesn't solve the must-name-the-base-class problem, but it helps
> manage it better.  If you keep the override assignments at the top of
> the class statement, then you only have one place to look in each
> class when the inheritance hierarchy changes.

Cute!  You can also make it a mixin, along the lines of:

import new
class Mixin:
    def __getattr__(self, k):
	if k[:len('super_')] == 'super_':
	    for b in self.__class__.__bases__:
		m = getattr(b, k[len('super_'):], None)
		if m: 
		    return new.instancemethod(m, self, self.__class__)
	    else:
		raise AttributeError, k
	else:
	    raise AttributeError, k

class Base(Mixin):
    def f(self): 
	print 'Base.f'
    
class Sub(Base):
    def f(self):
	print 'Sub.f'
	self.super_f()


s = Sub()
s.f()


Not especially fast, of course, but doesn't require as much human memory
load. 

--david





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