variable length tuple assignment

Sorry if this is too simple but I couldn't find.

I vaguely remember there is a means to assign a variable length tuple
and catch the 'rest' like

S="a,b,c,d"

(A,B,<list of remaining items>) = S.split(',')

I know I could do
SL= split(',')
(A,B)=SL[:2]
Rest= SL[2:]

but is there some shorthand for this?

Many thanks for a hint,

Helmut Jarausch

Lehrstuhl fuer Numerische Mathematik
RWTH - Aachen University
D 52056 Aachen, Germany

Helmut Jarausch

On Wed, Feb 25, 2009 at 1:16 AM, Helmut Jarausch
<> wrote:
> Sorry if this is too simple but I couldn't find.
>
> I vaguely remember there is a means to assign a variable length tuple
> and catch the 'rest' Â*like
>
> S="a,b,c,d"
>
> (A,B,<list of remaining items>) = S.split(',')

In Python 3.0 (IIRC):

A, B, *rest = S.split(',')

Cheers,
Chris

--
Follow the path of the Iguana...
http://rebertia.com

Chris Rebert

Chris Rebert wrote:
> On Wed, Feb 25, 2009 at 1:16 AM, Helmut Jarausch
> <> wrote:
>> Sorry if this is too simple but I couldn't find.
>>
>> I vaguely remember there is a means to assign a variable length tuple
>> and catch the 'rest' like
>>
>> S="a,b,c,d"
>>
>> (A,B,<list of remaining items>) = S.split(',')
>
> In Python 3.0 (IIRC):
>
> A, B, *rest = S.split(',')

As an aside, as of the last time I read the PEP[1] on this, I
believe it exhausts (or attempts to exhaust) any iterator. IMHO,
I think this exhausting is a bad idea because it prevents things like

def numbers(start=0):
i = start
while True:
yield i
i += 1

CONST_A, CONST_B, CONST_C, *rest = numbers()

which will hang in current Py3.0 until you blow a stack or
overrun your heap somewhere because it will try to exhaust the
infinite loop. It also changes the type from iter() to list()
for the remaining content (not as grevious).


I agree that the "internal star" usage needs to exhaust the iterator:

A, *rest, C, D = s.split(',')

but the terminal-star syntax should be a little more gracious
with iterators.

Anyways, my $0.02 (minus taxes, money for multiple bailouts, and
deflated by economic conditions -- so not worth much .

-tkc

[1]
http://www.python.org/dev/peps/pep-3132/

Tim Chase

Tim Chase wrote:

> As an aside, as of the last time I read the PEP[1] on this, I
> believe it exhausts (or attempts to exhaust) any iterator. IMHO,
> I think this exhausting is a bad idea because it prevents things like
>
> def numbers(start=0):
> i = start
> while True:
> yield i
> i += 1
>
> CONST_A, CONST_B, CONST_C, *rest = numbers()
>
> which will hang in current Py3.0 until you blow a stack or
> overrun your heap somewhere because it will try to exhaust the
> infinite loop.

But what else can it do? Do you expect Python to read your mind and
magically know when you intend to use rest and when you're intending to
just throw it away?

Perhaps Python could do that, via static analysis -- if rest is never used
again, don't bother exhausting the iterator. But that will lead to
differences in iterators that have side-effects. It will also have a subtle
difference in behaviour here:

it = xrange(5) # for example
a, b, c, *rest = it
L = list(it) # L is now [3, 4]

versus

a, b, c, *rest = xrange(5)
parrot(rest)
L = list(it) # L is now []


> It also changes the type from iter() to list()
> for the remaining content (not as grevious).

But that's what unpacking does. It would be a major semantic change for

x, *s = some_iterator

to make s an alias of some_iterator. And why would you want it to? If you
do, just do this:

x, s = some_iterator.next(), some_iterator



--
Steven

Steven D'Aprano