[Python-ideas] Explicit self argument, implicit super argument

Neil Toronto ntoronto at cs.byu.edu
Mon Nov 19 21:42:16 CET 2007

Guido van Rossum wrote:
> The reason for explicit self in method definition signatures is
> semantic consistency. If you write
> class C:
>   def foo(self, x, y): ...
> This really *is* the same as writing
> class C:
>   pass
> def foo(self, x, y): ...
> C.foo = foo
> And of course it works the other way as well: you really *can* invoke
> foo with an explicit argument for self as follows:
> class D(C):
>   ...
> C.foo(D(), 1, 2)
> IOW it's not an implementation hack -- it is a semantic device.

Ah, thanks, that helps. (I'll be able to  sleep tonight. :D) This 
semantic device, of course, would really suck if applied to "super":

d = D()
C.foo(d, super(C, d), 1, 2)  # strange and hideous

which is a great reason that the new "super" is implicit.

(Before I continue, please understand that I'm not arguing for a 
language change. Responses to my last two ideas have shown me that I 
need to thoroughly understand why things are as they are right now while 
considering a change, and long before advocating one. It also goes over 
better with the language designers.)

Now, correct me if I'm wrong, but it seems there are only two use cases 
for DistantParentOfD.method(D_instance, ...):

1. The Good Case: you know the "next-method" as determined by the MRO 
isn't the right one to call. Multiple inheritance can twist you into 
this sort of behavior, though if it does, your design likely needs 

2. The Evil Case: you know the override method as defined by D isn't the 
one you want for your extra-special D instance. This should be possible 
but never encouraged.

Because the runtime enforces isinstance(D_instance, D), everything else 
can be handled with D_instance.method(...) or self.method() or 
super.method(). We know that #1 and #2 above are the uncommon cases, 
which is why the new "super", which covers the common ones, doesn't 
cover those.

Is it right to say that the explicit "self" parameter only exists to 
enable those two uncommon cases?

Of course, if self were implicit, there would still need to be a way to 
spell DistantParentOfD.method(D_instance, ...). Being the uncommon case, 
maybe it shouldn't have a nice spelling:

as_parent(C, D_instance).method(...)



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