tp_base, ob_type, and tp_bases

[Jeff McNeil]

On Jan 17, 11:09 am, Jeff McNeil <j... at jmcneil.net> wrote:
> On Jan 17, 10:50 am, "Martin v. Löwis" <mar... at v.loewis.de> wrote:
>
>
>
> > > So, the documentation states that ob_type is a pointer to the type's
> > > type, or metatype. Rather, this is a pointer to the new type's
> > > metaclass?
>
> > That's actually the same. *Every* ob_type field points to the object's
> > type, e.g. for strings, integers, tuples, etc. That includes type
> > objects, where ob_type points to the type's type, i.e. it's meta-type,
> > also called metaclass (as "class" and "type" are really synonyms).
>
> > > Next, we have tp_base.  That's defined as "an optional pointer to a
> > > base type from which type properties are inherited."  The value of
> > > tp_base is then added to the tp_bases tuple.  This is confusing me. On
> > > the surface, it sound as though they're one in the same?
>
> > (I don't understand the English "one in the same" - interpreting it
> > as "as though they should be the same")
>
> > No: tp_bases is a tuple of all base types (remember, there is multiple
> > inheritance); tp_base (if set) provides the first base type.
>
> > > I *think* (and dir() of a subclass of type seems to back it up), that
> > > tp_base is only at play when the object in question inherits from
> > > type?
>
> > No - it is the normal case for single inheritance. You can leave it
> > NULL, which means you inherit from object.
>
> > Regards,
> > Martin
>
> Thank you! It was tp_base that was confusing me.  The tp_bases member
> makes sense as Python supports multiple inheritance.  It wasn't
> immediately clear that tp_base is there for single inheritance
> reasons. It's all quite clear now.
>
> Is that an optimization of sorts?
>
> Jeff

Well, maybe not specifically for single inheritance reasons, I just
didn't see an immediate reason to keep a separate pointer to the first
base type.