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Re: Differences creating tuples and collections.namedtuples

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On 19 February 2013 00:18, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> Terry Reedy wrote:
>> On 2/18/2013 6:47 AM, John Reid wrote:
[snip]
>>> Is this a problem with namedtuples, ipython or just a feature?
>>
>> With canSequence. If isinstance was available and the above were written
>> before list and tuple could be subclassed, canSequence was sensible when
>> written. But as Oscar said, it is now a mistake for canSequence to
>> assume that all subclasses of list and tuple have the same
>> initialization api.
>
> No, it is not a mistake. It is a problem with namedtuples that they violate
> the expectation that they should have the same constructor signature as
> other tuples. After all, namedtuples *are* tuples, they should be
> constructed the same way. But they aren't, so that violates a reasonable
> expectation.

It is a mistake. A namedtuple class instance provides all of the
methods/operators provided by a tuple. This should be sufficient to
fill the tuplishness contract. Requiring that obj satisfy a contract
is one thing. When you get to the point of requiring that type(obj)
must do so as well you have gone beyond duck-typing and the normal
bounds of poly-morphism.

It's still unclear what the purpose of canSequence is, but I doubt
that there isn't a better way that it (and its related functions)
could be implemented that would not have this kind of problem.


Oscar

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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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