[Python-3000] Using *a for packing in lists and other places

[Phillip J. Eby]

At 06:33 PM 3/15/2008 -0500, Michael Urman wrote:
> > > I'm not sure how to solve this except by adopting a different syntax
> > > for the multiple-yield. Perhaps
> > >
> > > *yield x
> >
> > If there really were an inconsistency here, I would certainly not suggest
> > fixing it that way, yuuueghh.
>
>Agreed about *yield looking yucky. In a previous thread somewhere
>around http://mail.python.org/pipermail/python-dev/2006-January/059955.html
>a construct like "yield from x" was offered. It certainly helps
>clarify the difference between iteratively yielding the values from
>another iterator and yielding the values of an iterator as a tuple.
>While parsing rules do differentiate, I don't find the behavior
>obvious, and expect the differences between
>     yield *x; yield *x(); yield *x,; yield (*x,); and yield *(x,)
>would become one of python's warts in a few years, if adopted.

After following this thread for a bit, I think it's a wart 
already.  :)  The potential for confusion with the other *assign 
target idea is too high.

I don't have a problem with "item for *item,val in blah", but "*item 
for item in blah" makes my head hurt.  Why not just say "i2 for i1 in 
blah for i2 in i1"?  Yes, it's a bit more verbose, but not frequent 
enough to justify the *, and you can tell what's going on by 
textually transposing to the nested for loops.  Plus, it works for 
any depth.  What are you going to do if it's triple-nested?  Use 
**?  What about ***?  Where does it end?

So I think "yield *" and "*item for item in..." are broken for the 
same reason.  In the function call use case, and in simple 
assignment, it is treated as a literal quasi-textual 
expansion.  However, such an expansion in the yield case would yield 
a tuple, and the literal expansion in the comprehension case would 
make no sense.  (If done by true textual substitution, it would have 
to be a syntax error.)

And I think that a quasi-textual substitution is the right mental 
model for * expressions, as it's simple to explain and easy to reason 
out what happens.