[Python-Dev] very slow compare of recursive objects

[Jeff Epler]

On Mon, Jan 20, 2003 at 10:32:50AM -0500, Tim Peters wrote:
> [Tim]
> >Tuples can't be recursive,
> 
> [Guido]
> > Oh yes they can be:
> >
> >    >>> L = []
> >    >>> t = (L, L)
> >    >>> L.append(L)
> >    >>>
> 
> That's an example of a tuple *containing* a recursive structure; there's
> still no way, starting at t, to get back to t.  I think mutation is required
> for that.  Like
> 
>     L = [None]
>     t = (L, L)
>     L[0] = t
> 
> Then
> 
>     t[0]0] is t
> 
> and that's what I mean by "recursive tuple" (a tuple that can be reached
> from itself).

Is this an example of the elusive "recursive tuple"?

class X(tuple):
    def __getitem__(self, idx):
        if idx == 0: return self
        return tuple.__getitem__(self, idx)
        
    def __len__(self):
        return min(1, tuple.__len__(self))

>>> x = X([None])
>>> print len(x), x[0] is x, x==x
1 True True
>>> print x
(None,)

I'm also a bit confused by the last line.  I guess the builtin
tuple.__repr__ uses the C-level API to access tuple items, not the real
__getitem__ slot?  pprint shows the same thing, though.

Jeff