[Python-Dev] very slow compare of recursive objects (original) (raw)

Jeff Epler jepler@unpythonic.net
Mon, 20 Jan 2003 09:53:15 -0600

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