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

Tim Peters tim.one@home.com
Sat, 5 May 2001 16:40:11 -0400

[Guido]

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),

Ya, it sucks. I was inspired by that instance_contains() itself makes dubious assumptions about what an AttributeError means when the functions it calls raise it .

but to reimplement the default behavior in the instance slot implementation.

The "backward compatibility" comment in instance_contains() was scary: compatibility with what? instance_contains() is pretty darn new. I assumed it meant there was some good (but unidentified) reason we had to use PyObject_Cmp() instead of PyObject_RichCompareBool(..., Py_EQ) if instance_item() "worked". But I haven't thought of one, except to ensure that

some_complex  in  some_instance_with___getitem__

continues to blow up -- but that's not a good reason. So:

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

Will do!