[Python-Dev] PyInstance_Check() and new-style classes (original) (raw)

Eric Wilhelm ewilhelm at sbcglobal.net
Mon Jul 12 16:01:08 CEST 2004

The following was supposedly scribed by

Michael Hudson

on Monday 12 July 2004 08:08 am:

tell me if I'm doing this "the right way" (TM). Why does it matter?  I did actually read the rest of your post, but this failed to leap out at me (apologies if I'm being dumb).

For starters, the process-of-elimination way of determining the type is pretty clunky, and only works on classes which have subclassed the 'object' type (not lists, tuples, strings, etc.)

Also, it seems that the type/class unification has broken the API function PyInstance_Check(), which I think should tell me if I'm dealing with an instance of a builtin type. If not, it seems that there should be some function which allows me to perform this check.

There's no real answer -- there's just not that much difference between user-defined new-style classes and builtin types (part of their appeal!) but checking the TPHEAPTYPE flag in tpflags may go some way towards one.

I'll have to look into this. I'm not sure what that would tell me.

Basically, Perl's "objects" are just bless()ed references. That seems analogous to the direction that Python has gone with the type/class unification, but Perl is a little less formal about objects.

This brings up another problem with my current code in that a subclass of an actual builtin type (as opposed to just 'object') needs to be handled differently when constructing the perl values. e.g. I'd like to return a blessed reference to a Perl array if the class is subclassing the python array. This won't work with the current scheme, since the blessed reference for a class instance is a reference to a scalar containing its memory address.

However, I'm willing to punt that and just use the pointer to the python object (be it a list, tuple, or object) provided that I can identify it as a class instance.

But, this won't work with the way things are now, since PySequence_Check() would presumably return true for an instance of a class which had subclassed the builtin list type.

Granted, we could just pass pointers to python objects for everything, but that wouldn't make a very good perl binding, since we'd then have to use string−>append("foo")insteadofPerl′sbuiltinstring->append("foo") instead of Perl's builtin string−>append("foo")insteadofPerl′sbuiltinstring .= "foo" and I'm not sure yet how we would get to the actual value of $string under that schema.

--Eric

"Because understanding simplicity is complicated." --Eric Raymond

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