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

Nick Coghlan ncoghlan at iinet.net.au
Sat Jan 8 08:15:23 CET 2005

Ilya Sandler wrote:

How about making offset a standard integer, and change the signature to return tuple when it is used: item, offset = unpack(format, rec, offset) # Partial unpacking

Well, it would work well when unpack results are assigned to individual vars: x,y,offset=unpack( "ii", rec, offset) but it gets more complicated if you have something like: coords=unpack("10i", rec) How would you pass/return offsets here? As an extra element in coords? coords=unpack("10i", rec, offset) offset=coords.pop() But that would be counterintuitive and somewhat inconvinient..

I was thinking more along the lines of returning a 2-tuple with the 'normal' result of unpack as the first element:

coords, offset = unpack("ii", rec, offset) x, y = coords

Raymond's suggestion of a separate function like 'unpack_here' is probably a good one, as magically changing function signatures are evil. Something like:

def unpack_here(format, record, offset = 0): end = offset + calcsize(format) return (unpack(format, record[offset:end]), end)

Presumably, a C version could avoid the slicing and hence be significantly more efficient.

Yes, the return type is a little clumsy, but it should still make it easier to write more efficient higher-level API's that unpack the structure a piece at a time.

Cheers, Nick.

-- Nick Coghlan | ncoghlan at email.com | Brisbane, Australia

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