Paul Moore <[EMAIL PROTECTED]> writes:
> On Thu, 6 Jan 2005 21:28:26 +1100, Anthony Baxter
> <[EMAIL PROTECTED]> 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:
[...]
For reading structures from files, the undocumented (*) readinto
method is very nice. An example:
class IMAGE_DOS_HEADER(Structure):
....
class IMAGE_NT_HEADERS(Structure):
....
class PEReader(object):
def read_image(self, pathname):
################
# the MSDOS header
image = open(pathname, "rb")
self.dos_header = IMAGE_DOS_HEADER()
image.readinto(self.dos_header)
################
# The PE header
image.seek(self.dos_header.e_lfanew)
self.nt_headers = IMAGE_NT_HEADERS()
image.readinto(self.nt_headers)
> 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)
Maybe, but I would prefer the unbeloved buffer object (*) as argument,
because it has builtin offset and length.
> 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)
Correct.
(*) Which brings me to the questions I have in my mind for quite some
time: Why is readinto undocumented, and what about the status of the
buffer object: do the recent fixes to the buffer object change it's
status?
Thomas
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