[issue2292] Missing *-unpacking generalizations

[Thomas Wouters]

Thomas Wouters <thomas at python.org> added the comment:

On Mon, Apr 7, 2008 at 9:00 PM, Alexander Belopolsky <report at bugs.python.org>
wrote:

>
> Alexander Belopolsky <belopolsky at users.sourceforge.net> added the comment:
>
> Thomas,
>
> Could you add BUILD_*_UNPACK opcodes documentation to
> Doc/library/dis.rst?   It would also help if you modify CALL_FUNCTION_*
> opcodes' documentation to explain how they will interact with unpacking
> opcodes.

They don't interact. They're separate opcodes. The CALL_FUNCTION_* opcodes
are almost untouched, except the _VAR and _VAR_KW versions: previously, they
expected, on the stack, positional arguments followed by keyword/argument
pairs followed by the *args sequence followed by the **kwargs mapping (for
_VAR_KW.) I just changed the order so *args is before the keyword/argument
pairs. The change is not related to the BUILD_*_UNPACK opcodes, but rather
to Guido's request that the order of keyword arguments and *args in the
functioncall syntax changes. For the order of execution to remain sane, the
arguments need to be pushed on the stack in that order, and keeping the
_VAR* opcode order the same would mean a large amount of ROT_* opcodes ;-P

Updating the docs is on the TODO list.

>
> Do I understand correctly that non-starred arguments are packed into
> intermediate tuples/sets in the presence of starred arguments so that
> {a,b,*c,d,e} is equivalent to {*{a,b},*c,*{d,e}}? This should not be a
> problem for tuples, but with sets, it means that {a,b,c} may behave
> subtly differently from {a,*(b,c)}.
>

Yes, that's what happens, but only in the presence of *args. For
functioncalls, it only happens to everything after the first *args
(inclusive.) That means '{a, b, c}' does not change, and neither does
'func(a, b, c)' or 'func(a, b, *c)'. As for sets, I don't see why this would
be a problem; there is no difference in the set created by {a, b, c} and the
set created by {a, *{b, c}} or {a, *(b, c)}.  The arguments are all
evaluated in the same order (left to right), and c replaces b, b replaces a
if they are considered equal by sets.

Added file: http://bugs.python.org/file9970/unnamed

__________________________________
Tracker <report at bugs.python.org>
<http://bugs.python.org/issue2292>
__________________________________
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: unnamed
Url: http://mail.python.org/pipermail/python-bugs-list/attachments/20080407/4483c782/attachment-0001.ksh