[Martin]
> I think the error message could be better, though; it should probably be a ValueError.

Do you mean a ValueError at the level of the struct module, or the zipfile module.

I'd quite like to change all the exceptions raised by the struct module to ValueError or TypeError (as appropriate);  I'm not sure what the point of struct.error is supposed to be.  I've resisted making this change up until now for backwards compatibility reasons, but perhaps it could be considered for 3.2 (but not for 2.7).