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

Guido van Rossum gvanrossum at gmail.com
Sat Jan 8 17:52:31 CET 2005

First, let me say two things:

(a) A higher-level API can and should be constructed which acts like a (binary) stream but has additional methods for reading and writing values using struct format codes (or, preferably, somewhat higher-level type names, as suggested). Instances of this API should be constructable from a stream or from a "buffer" (e.g. a string).

(b) -1 on Ilya's idea of having a special object that acts as an input-output integer; it is too unpythonic (no matter your objection).

[Paul Moore]

OTOH, Nick's idea of returning a tuple with the new offset might make your example shorter without sacrificing readability:

_result, newpos = struct.unpack('>l', self._buf, self.pos) _self.pos = newpos # retained "newpos" for readability... return result

This is okay, except I don't want to overload this on unpack() -- let's pick a different function name like unpack_at().

A third possibility - rather than "magically" adding an additional return value because you supply a position, you could have a "where am I?" format symbol (say & by analogy with the C "address of" operator). Then you'd say

_result, newpos = struct.unpack('>l&', self._buf, self.pos) Please be aware, I don't have a need myself for this feature - my interest is as a potential reader of others' code...

I think that adding more magical format characters is probably not doing the readers of this code a service.

I do like the idea of not introducing an extra level of tuple to accommodate the position return value but instead make it the last item in the tuple when using unpack_at().

Then the definition would be:

def unpack_at(fmt, buf, pos): size = calcsize(fmt) end = pos + size data = buf[pos:end] if len(data) < size: raise struct.error("not enough data for format") # if data is too long that would be a bug in buf[pos:size] and cause an error below ret = unpack(fmt, data) ret = ret + (end,) return ret

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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