[Python-Dev] Subclassing varying length types (What's a PyStructSequence ?)

[M.-A. Lemburg]

"Martin v. Loewis" wrote:
> 
> > The discussion on SF doesn't really answer my question. What
> > Nick did is fascinating: he reused the type object implementation
> > to mimic a sequence ! That's cool, but looks like an awfully
> > tricky way of doing something straight forward such as sub-classing
> > the tuple type to extend it with an additional dictionary.
> > So the question remains: why did Nick *have* to implement this
> > as meta-type ?
> 
> For one thing, you'll see from the discussion that extending the tuple
> type with an additional dict is non-trivial: You cannot define a C
> data type that does this. You'll also see that there was a version
> that did it, and that it was rejected precisely because of this problem.

Ok, now we're getting somewhere... you're saying that Python types
using the PyObject struct itself to store variable size data
cannot be subclassed in C. Even though it's not trivial as
you indicate, this should well be possible by appending new
object data to the end of the allocated data field -- not very
elegant, but still a way to cope with the problem.

However, Nick's approach of creating a new type from a template
by using the fact that the list of known name-to-index mappings
is not going to change for instances of the type makes things
a lot cleaner, since now the mapping can live in the
type definition rather than the instance (which may very
well be a varying length PyObject type).

Hmm, this makes me wonder: perhaps we should start thinking
about phasing out varying length PyObjects in the interpreter... 
esp. the inability to subclass strings looks like a bummer for 
future extensions of this particular type. Unicode doesn't have
this problem, BTW.

Or we need to come up with a fairly nice way of making 
subclassing varying length types a lot easier, e.g. by
adding a special pointer ob_ext to PyObject_VAR_HEAD 
which then allows declaring type extensions in an malloced 
buffer.

Thoughts ?

-- 
Marc-Andre Lemburg
CEO eGenix.com Software GmbH
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