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Re: Differences creating tuples and collections.namedtuples

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On 02/18/2013 06:47 AM, John Reid wrote:
> Hi,
>
> I was hoping namedtuples could be used as replacements for tuples in all instances. There seem to be some differences between how tuples and namedtuples are created. For example with a tuple I can do:
>
> a=tuple([1,2,3])
>
> with namedtuples I get a TypeError:
>
> from collections import namedtuple
> B=namedtuple('B', 'x y z')
> b=B([1,2,3])

You are passing a single list to the constructor, but you specified that 
the namedtuple was to have 3 items.  So you need two more.

>
> TypeError: __new__() takes exactly 4 arguments (2 given)
>> <ipython-input-23-d1da2ef851fb>(3)<module>()
>        1 from collections import namedtuple
>        2 B=namedtuple('B', 'x y z')
> ----> 3 b=B([1,2,3])
>
> I'm seeing this problem because of the following code in IPython:
>
> def canSequence(obj):
>      if isinstance(obj, (list, tuple)):
>          t = type(obj)
>          return t([can(i) for i in obj])
>      else:
>          return obj
>
> where obj is a namedtuple and t([can(i) for i in obj]) fails with the TypeError. See http://article.gmane.org/gmane.comp.python.ipython.user/10270 for more info.
>
> Is this a problem with namedtuples, ipython or just a feature?
>
> Thanks,
> John.
>

If you want one item (list or tuple) to act like 3 separate arguments, 
you could use the "*" operator:

b = B( *[1,2,3] )

or in your canSequence function, if you want a namedTuple
           return t(*[can(i) for i in obj])



-- 
DaveA

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Differences creating tuples and collections.namedtuples John Reid <johnbaronreid@gmail.com> - 2013-02-18 03:47 -0800
  Re: Differences creating tuples and collections.namedtuples Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-02-18 12:03 +0000
  Re: Differences creating tuples and collections.namedtuples Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-02-18 12:05 +0000
  Re: Differences creating tuples and collections.namedtuples Dave Angel <davea@davea.name> - 2013-02-18 07:11 -0500
  Re: Differences creating tuples and collections.namedtuples John Reid <johnbaronreid@gmail.com> - 2013-02-18 13:49 +0000
  Re: Differences creating tuples and collections.namedtuples John Reid <johnbaronreid@gmail.com> - 2013-02-18 13:51 +0000
  Re: Differences creating tuples and collections.namedtuples John Reid <j.reid@mail.cryst.bbk.ac.uk> - 2013-02-18 14:09 +0000
    Re: Differences creating tuples and collections.namedtuples raymond.hettinger@gmail.com - 2013-02-18 23:48 -0800
      Re: Differences creating tuples and collections.namedtuples Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-19 08:06 +0000
      Re: Differences creating tuples and collections.namedtuples Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-19 08:57 +0000
      Re: Differences creating tuples and collections.namedtuples Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-19 08:06 +0000
      Re: Differences creating tuples and collections.namedtuples Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-19 08:05 +0000
    Re: Differences creating tuples and collections.namedtuples raymond.hettinger@gmail.com - 2013-02-18 23:48 -0800
  Re: Differences creating tuples and collections.namedtuples Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-02-18 14:15 +0000
  Re: Differences creating tuples and collections.namedtuples John Reid <johnbaronreid@gmail.com> - 2013-02-18 14:18 +0000
  Re: Differences creating tuples and collections.namedtuples Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-02-18 14:12 +0000
  Re: Differences creating tuples and collections.namedtuples John Reid <j.reid@mail.cryst.bbk.ac.uk> - 2013-02-18 14:23 +0000
  Re: Differences creating tuples and collections.namedtuples Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-02-18 14:53 +0000
  Re: Differences creating tuples and collections.namedtuples John Reid <j.reid@mail.cryst.bbk.ac.uk> - 2013-02-18 15:07 +0000
  Re: Differences creating tuples and collections.namedtuples Terry Reedy <tjreedy@udel.edu> - 2013-02-18 16:28 -0500
    Re: Differences creating tuples and collections.namedtuples Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-19 11:18 +1100
      Re: Differences creating tuples and collections.namedtuples Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-02-19 01:43 +0000
        Re: Differences creating tuples and collections.namedtuples Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-19 14:06 +1100
          Re: Differences creating tuples and collections.namedtuples Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2013-02-19 21:27 +1300
      Re: Differences creating tuples and collections.namedtuples Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2013-02-19 20:54 +1300
      Re: Differences creating tuples and collections.namedtuples John Reid <j.reid@mail.cryst.bbk.ac.uk> - 2013-02-19 09:30 +0000
      Re: Differences creating tuples and collections.namedtuples Terry Reedy <tjreedy@udel.edu> - 2013-02-19 22:38 -0500
        Re: Differences creating tuples and collections.namedtuples Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-22 10:38 +0000
      Re: Differences creating tuples and collections.namedtuples Chris Angelico <rosuav@gmail.com> - 2013-02-20 17:50 +1100
        Re: Differences creating tuples and collections.namedtuples Roy Smith <roy@panix.com> - 2013-02-20 09:09 -0500
    Re: Differences creating tuples and collections.namedtuples Roy Smith <roy@panix.com> - 2013-02-18 20:11 -0500
  Re: Differences creating tuples and collections.namedtuples alex23 <wuwei23@gmail.com> - 2013-02-18 17:47 -0800
    Re: Differences creating tuples and collections.namedtuples John Reid <j.reid@mail.cryst.bbk.ac.uk> - 2013-02-19 09:36 +0000

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