On 2017-01-19 12:47, Elizabeth Myers wrote:
On 19/01/17 05:58, Rhodri James wrote:
On 19/01/17 08:31, Mark Dickinson wrote:
On Thu, Jan 19, 2017 at 1:27 AM, Steven D'Aprano <st...@pearwood.info>
wrote:
[...] struct already supports
variable-width formats.
Unfortunately, that's not really true: the Pascal strings it supports
are in some sense variable length, but are stored in a fixed-width
field. The internals of the struct module rely on each field starting
at a fixed offset, computable directly from the format string. I don't
think variable-length fields would be a good fit for the current
design of the struct module.
For the OPs use-case, I'd suggest a library that sits on top of the
struct module, rather than an expansion to the struct module itself.
Unfortunately as the OP explained, this makes the struct module a poor
fit for protocol decoding, even as a base layer for something. It's one
of the things I use python for quite frequently, and I always end up
rolling my own and discarding struct entirely.
Yes, for variable-length fields the struct module is worse than useless:
it actually reduces clarity a little. Consider:
test_bytes = b'\x00\x00\x00\x0chello world!'
With this, you can do:
length = int.from_bytes(test_bytes[:4], 'big')
string = test_bytes[4:length]
Shouldn't that be:
string = test_bytes[4:4+length]
or you can do:
length = struct.unpack_from('!I', test_bytes)[0]
string = struct.unpack_from('{}s'.format(length), test_bytes, 4)[0]
Which looks more readable without consulting the docs? ;)
Which is more likely to be correct? :-)
Building anything on top of the struct library like this would lead to
worse-looking code for minimal gains in efficiency. To quote Jamie
Zawinksi, it is like building a bookshelf out of mashed potatoes as it
stands.
If we had an extension similar to netstruct:
length, string = struct.unpack('!I$', test_bytes)
MUCH improved readability, and also less verbose. :)
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