[Python-Dev] an idea for improving struct.unpack api

[Paul Moore]

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.