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Groups > comp.lang.python > #39191

Re: Differences creating tuples and collections.namedtuples

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Steven D'Aprano wrote:
> py> class MyDict(dict):
> ...     @classmethod
> ...     def fromkeys(cls, func):
> ...         # Expects a callback function that gets called with no arguments
> ...         # and returns two items, a list of keys and a default value.
> ...         return super(MyDict, cls).fromkeys(*func())

Here you've overridden a method with one having a
different signature. That's not something you'd
normally do, because, being a method, it's likely
to get invoked polymorphically.

Constructors, on the other hand, are usually *not*
invoked polymorphically. Most of the time we know
exactly which constructor we're calling, because we
write the class name explicitly at the point of call.

Consequently, we have a different attitude when it
comes to constructors. We choose not to require LSP
for constructors, because it turns out to be very
useful not to be bound by that constraint.
Practicality beats purity here.

The reason IPython gets into trouble is that it tries
to make a polymorphic call to something that nobody
expects to need to be polymorphic.

-- 
Greg

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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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