[Python-Dev] an idea for improving struct.unpack api (original) (raw)

Paul Moore p.f.moore at gmail.com
Thu Jan 6 12:38:39 CET 2005

On Thu, 6 Jan 2005 21:28:26 +1100, Anthony Baxter <anthony at interlink.com.au> wrote:

My take on this:

struct.pack/struct.unpack is already one of my least-favourite parts of the stdlib. Of the modules I use regularly, I pretty much only ever have to go back and re-read the struct (and re) documentation because they just won't fit in my brain. Adding additional complexity to them seems like a net loss to me.

Have you looked at Thomas Heller's ctypes? Ignoring the FFI stuff, it has a fairly comprehensive interface for defining and using C structure types. A simple example:

class POINT(Structure): ... fields = [('x', c_int), ('y', c_int)] ... p = POINT(1,2) p.x, p.y (1, 2) str(buffer(p)) '\x01\x00\x00\x00\x02\x00\x00\x00'

To convert from a byte string is messier, but not too bad:

s = str(buffer(p)) s '\x01\x00\x00\x00\x02\x00\x00\x00' p2 = POINT() ctypes.memmove(p2, s, ctypes.sizeof(POINT)) 14688904 p2.x, p2.y (1, 2)

It might even be possible to get Thomas to add a small helper classmethod to ctypes types, something like

POINT.unpack(str, offset=0, length=None)

which does the equivalent of

def unpack(cls, str, offset=0, length=None):
    if length is None:
        length=sizeof(cls)
    b = buffer(str, offset, length)
    new = cls()
    ctypes.memmove(new, b, length)
    return new

I'd love to find the time to write a sane replacement for struct - as well as the current use case, I'd also like it to handle things like attribute-length-value 3-tuples nicely (where you get a fixed field which identifies the attribute, a fixed field which specifies the value length, and a value of 'length' bytes). Almost all sane network protocols (i.e. those written before the plague of pointy brackets) use this in some way.

I'm not sure ctypes handles that, mainly because I don't think C does (without the usual trick of defining the last field as fixed length)

Paul.

More information about the Python-Dev mailing list