[Python-Dev] What's a PyStructSequence ?

[M.-A. Lemburg]

nickm@alum.mit.edu wrote:
> 
> Marc-Andre Lemburg wrote:
> > "Martin v. Loewis" wrote:
> > > > > A bug report on SF made me aware of an apparently new type in Python
> > > > > called PyStructSequence. There are no docs on the type (at least
> > > > not in the usual places).
> > > >
> > > > Is it official yet ?
> > > > It will ship as part of Python 2.2, if that is what you are
> > > asking. os.stat is documented to return one of these (if you read it
> > > carefully).
> >
> >Wouldn't it make sense to expose this object in Python,
> >e.g. by contructing it from a dictionary of string mappings ?
> >
> >(The type constructor is not made available in bltinmodule.c.)
> 
> Hi, all.  I'm not subscribed to python-dev, but I'm the author of the
> original patch, and I thought I should comment.
> 
> If you look closely, you'll find that PyStructSequence is not a type
> itself, but rather a tool used to construct new tuple/struct hybrid
> types, like the results of os.stat and time.gmtime.

Indeed -- and I have a question there: why did you have to implement
this as meta-type ? It seems that the same thing could have been
done using a normal type which then gets initialized after
instantiation. Or was it to get used to the new type system :-?
 
> In reality, PyStructSequence is only a set of common implementation
> logic for a set of other types, including os.stat_result,
> os.statvfs_result, and time.struct_time.
> 
> There are a few possible objections to this scheme:
> 
>       Q. Nick, why didn't you make it a _real_ metatype?
> 
>       A. Writing a real metatype in C was beyond my Python
>          abilities. If anybody wants to, I'd be thrilled.a
> 
>     Q. Okay, so why not expose it to python?
> 
>     A. Because it isn't a real metatype.  Every type that uses it _is_
>        exposed to python.
> 
>        I think this isn't a problem, because it's way easier to
>        re-implement PyStructSequence in Python than it is to turn it
>        into a metatype.
> 
>     Q. If it's so easy to write in Python, why not do it that way?
> 
>     A. Because there are fringe benefits to doing it in C.
> 
>        For example, on some Unix machines (such as Linux), struct stat
>        has some attributes that don't correspond to any elements of
>        the old tuple view.  To expose (say) st_rdev to Python code at
>        all, you'd need to change the result of posix.stat... but this
>        would break code that used posix.stat directly.
> 
>        But because PyStructSequence is written in C, posix.stat can
>        return an augmented tuple/struct hybrid that (when accessed as
>        a tuple) still has 10 elements, but also exposes st_rdev as an
>        attribute.

This would have also been possible using the "normal" approach; 
I'm still not convinced -- it looks too much like an academic 
experiment ;-).

-- 
Marc-Andre Lemburg
CEO eGenix.com Software GmbH
______________________________________________________________________
Consulting & Company:                           http://www.egenix.com/
Python Software:                        http://www.lemburg.com/python/