Unpacking extension (Re: A small inconsistency in syntax?)

[Bernhard Herzog]

James_Althoff at i2.com writes:

> Greg Ewing wrote:
> >What if, instead of just the special built-in constructors
> >() and [], you were allowed *any* constructor on the LHS?
> >
> >Consider:
> >
> >  class Pair:
> >    def __init__(self, x, y):
> >      self.x = x
> >      self.y = y
> >    def __unpack__(self):
> >      return x, y
> >
> >  p = Pair(17, 42)
> >  Pair(a, b) = p

What's wrong with e.g.

class Pair:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def __getitem__(self, index):
        return (self.x, self.y)[index]

p = Pair(17, 42)
a, b = p


That works today (just tested with 2.1 and 1.5.2).

The only thing it doesn't do is checking whether p is an instance of
Pair as your proposal would but that's better done with a separate test,
anyway, IMO.

AFAIK, the "[a,b] = sequence" syntax is a holdover of some old python
version (<= 1.4 IIRC) where "a, b = sequence" would only work for tuples
and "[a,b] = sequence" only for lists. 

Personally, I wouldn't mind to see the "[a,b] = sequence" sytax go.

   Bernhard

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