[Python-Dev] very slow compare of recursive objects

[Guido van Rossum]

> Speaking of graceful approaches, I expect that this code:
> 
> 		(v->ob_type->tp_as_mapping
> 		 || (v->ob_type->tp_as_sequence
> 		     && !PyString_Check(v)
> 		     && !PyTuple_Check(v)))) {
> 
> no longer does what it intended to do for tuples.  Tuples can't be
> recursive,

Oh yes they can be:

   >>> L = []
   >>> t = (L, L)
   >>> L.append(L)
   >>> 

> so it intended to exempt tuples from the recursive-compare
> machinery.  But Michael Hudson added a non-NULL tp_as_mapping slot to tuples
> in rev 2.65 of tupleobject.c, so tuples are no longer exempted by the
> recursive-compare gimmick.  That makes the problem in 2.3 worse than it used
> to be (although it's never been zippy, due to clearing inprogress dict
> entries while they're still potentially useful).  So all the hacks I added
> (in my ugly patch) to keep check_recursion's tuple operations out of this
> code would be better done by restoring the "don't look at tuples at all"
> intent of the code above.

--Guido van Rossum (home page: http://www.python.org/~guido/)