variable length tuple assignment

Tim Chase python.list at tim.thechases.com
Wed Feb 25 12:35:06 CET 2009


Chris Rebert wrote:
> On Wed, Feb 25, 2009 at 1:16 AM, Helmut Jarausch
> <jarausch at igpm.rwth-aachen.de> 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/









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