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

[Terry Reedy]

"Guido van Rossum" <guido at python.org> wrote in message 
news:ca471dc20803150915l27750346g8655f596f5035c69 at mail.gmail.com...
| Thomas Wouters suggests some new syntax:

I see this as two suggestions:
1. Complete the extension of the validity of *expression syntax from 
function call/definition to expression/assignment.
2. Give *expression a related meaning in yield statements.

|
| http://bugs.python.org/issue2292
|
| >>> a, b, *c = range(5)
|
| >>> *a, b, c = a, b, *c
| >>> a, b, c
| ([0, 1, 2], 3, 4)
| >>> [ *a, b, c ]
| [0, 1, 2, 3, 4]

I understand 'f(*a)' to mean 'execute the expression as if the items of 
iterable a has been written literally in the code (with 'as if' 
optimizations allowed for special cases).  The above follow the same rule.
+1
A tutorial written for Py3 without regard to history should then introduce 
*expressions in examples like the above first, and only later the usage for 
functions, as if things had been this way all along.

| >>> L = [ a, (3, 4), {5}, {6: None}, (i for i in range(7, 10)) ]
| >>> [ *item for item in L ]
| [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Since above will become equivalent to list(gen_exp), I see this as being 
tied to yield *exp.

| Also, yielding everything from an iterator:
|
| >>> def flatten(iterables):
| ...     for it in iterables:
| ...         yield *it

Following the general rule above for *exp, that would be the same as yield
tuple(it).  But that is nearly useless, whereas the the implicit inner for 
loop meaning is quite useful, with, perhaps, a speedup over an explicit 
inner loop.  Since yield is already pretty magical,a bit more might not 
hurt.

But, ... what do you do with
    yield *a,b,c # a,b,c as above?
Yield a 5-tuple?  That would clash badly with 'yield *a' not yielding a 
3-tuple.
Raise an exception?  That establishes/requires a new grammatical 
category -- yield_expression.  But I would see this as preferable to the 
clash.

Terry Jan Reedy