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/