[Python-Dev] PySequence_Contains (original) (raw)

Guido van Rossum guido@digicool.com
Sat, 05 May 2001 07:24:19 -0500

In a checkin message, Tim wrote:

The full story for instance objects is pretty much unexplainable, because instancecontains() tries its own flavor of iteration-based containment testing first, and PySequenceContains doesn't get a chance at it unless instancecontains() blows up. A consequence is that somecomplexnumber in someinstance dies with a TypeError unless someinstance.class defines iter but does not define getitem.

This kind of thing happens everywhere -- instances always define all slots but using the slots sometimes fails when the corresponding foo doesn't exist. Decisions based on the presence or absence of a slot are therefore in general not reliable; the only exception is the decision to call the slot or not. The correct solution is not to catch AttributeError and pretend that the slot didn't exist (which would mask an AttributeError occurring inside the contains method if there was one), but to reimplement the default behavior in the instance slot implementation.

In this case, that means that PySequence_Contains() can be simplified (no need to test for AttributeError), and instance_contains() should fall back to a loop over iter(self) rather than trying to use instance_item().

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