Is this a super bug?

[Michele Simionato]

"Bjorn Pettersen" <BPettersen at NAREX.com> wrote in message news:<mailman.1050811779.16487.python-list at python.org>...

> <snip> this is the sequence that "should" happen:
> 
> 1  super(ff, f)[0]  # f == self
> 2  superinst(type:<class ff>, obj:f)[0]
> 3  si.  getitem  (0)
> 4    si.  getattr  ('  getitem  ')
> 5    .. return str.  dict  ['  getitem  '].  get  (f)
> 6    boundmethod(str.  getitem  , f)
> 7  bmeth(0)
> 8   'a'

I don't understand what you are trying to say here ... 

> well, the transition from 3 to 4 doesn't happen which surprised me a
> little. What surprised me more was that I couldn't get   getattribute  
> to be called either (neither in the class or its metaclass... I've only
> got 2.3a2, and I read a note about this not working before a4. Is this
> what is happening? If not, the only other reason I could see would be
> from the refMan 5.3.2, "The primary must evaluate to an object of a
> sequence or mapping type."  If that is the case, then I still think it's
> a super bug, i.e. there is no reason super should assume that it's not
> "going to be" a sequence, in fact the direct superclass of ff  is  a
> sequence. To fix it simply add:
> 
>   def   getitem  (self, v):
>       m = self.  getattr  ('  getitem  ')
>       return m(v)
> 
> to the definition of super.

You may do that, but I don't like it. I like the fact that

super(cls,self)[index]

is *not* equivalent to

super(cls,self).__getitem__(index)

The present behaviour is the natural one, when you realize that super
objects are attribute descriptors. In other words:

super(cls,self) is *not* self

In you case 'self' is a string and has a __getitem__ method; nevertheless,
since super(cls,self) is a descriptor and not a string, I would not expect 
it to have a __getitem__ method, nor the other methods of a string. 

What happens in reality (disclaimer: I don't know nothing about the C 
implementation, I am guessing here) is that super(cls,self) has a 
__getattribute__ method that call the methods of the string object: the 
fact that you can write

super(cls,self).__getitem__(index)

does *not* means that super(cls,self) has a __getitem__ method.

I agree this can be confusing at first. The solution would be to
document ``super`` and the attribute descriptors. 

Maybe one of these days I will collect my notes and fill this gap,
but this would require at least 20-30 pages, so it would take some
time ...


Hope this helps,

--
Michele Simionato - Dept. of Physics and Astronomy
210 Allen Hall Pittsburgh PA 15260 U.S.A.
Phone: 001-412-624-9041 Fax: 001-412-624-9163
Home-page: http://www.phyast.pitt.edu/~micheles/