I want to be able to reserve some storage in my own 
new-style class objects. Ideally the storage would 
fall before the variable-length section so I didn't 
have to worry about alignment issues.

-Dave

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I cannot fully understand the requirement. Are you talking
about a type implemented in C, or a class in Python?

Assuming it is a C type: Are you defining a type that
inherits from a builtin, or one that doesn't.

Assuming does not inherit: what is the problem with just
setting tp_basicsize correctly?

If it is a C type and it does inherit from a variable-length
builtin, this won't be possible: the API for the base type
won't know about the extra fields. In that case, extend the
type so that it has an __dict__ and put everything into the
dict.

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It's a C subtype of PyBaseObjectType, and you can't just 
set tp_basicsize correctly because the base type has its 
own idea of where the variable length section starts and 
doesn't respect a difference in tp_basicsize. I've spoken 
with Guido about this; he understands all the details. I 
entered this feature request in the tracker at his request 
(the weakref subclassing was entered at Fred's request).

Seems very out of date.

I'll close this unless there's a positive response within a couple of weeks.

I can't imagine what kind of "positive response" you'd want from me.  I responded to the last question asked.  I certainly don't know whether this is still an issue, though.