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

[Paul Moore]

On Fri, 7 Jan 2005 19:40:18 -0800 (PST), Ilya Sandler <ilya at bluefir.net> wrote:
> Eg. I just looked at xdrlib.py code and it seems that almost every
> invocation of struct._unpack would shrink from 3 lines to 1 line of code
> 
> (        i = self.__pos
>         self.__pos = j = i+4
>         data = self.__buf[i:j]
>         return struct.unpack('>l', data)[0]
> 
> would become:
>         return struct.unpack('>l', self.__buf, self.__pos)[0]
> )

FWIW, I could read and understand your original code without any
problems, whereas in the second version I would completely miss the
fact that self.__pos is updated, precisely because mutating arguments
are very rare in Python functions.

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

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...

Paul.