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From: Roy Smith <roy@panix.com>
Newsgroups: comp.lang.python
Subject: Re: Differences creating tuples and collections.namedtuples
Date: Wed, 20 Feb 2013 09:09:08 -0500
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In article <mailman.2087.1361343028.2939.python-list@python.org>,
 Chris Angelico <rosuav@gmail.com> wrote:

> On Wed, Feb 20, 2013 at 2:38 PM, Terry Reedy <tjreedy@udel.edu> wrote:
> > Liskov Substitution Principle (LSP): I met this over 15 years ago reading
> > debates among OOP enthusiasts about whether Rectangle should be a subclass
> > of Square or Square a subclass of Rectangle, and similarly, whether Ostrich
> > can be a legitimate subclass of Bird.
> >
> > The problem I see with the LSP for modeling either abstract or concrete
> > entities is that we in fact do define subclasses by subtraction or
> > limitation, as well as by augmentation, while the LSP only allows the
> > latter.
> 
> A plausible compromise is to demand LSP in terms of programming, but
> not necessarily functionality. So an Ostrich would have a fly() method
> that returns some kind of failure, in the same way that any instance
> of any flying-bird could have injury or exhaustion that prevents it
> from flying. It still makes sense to attempt to fly - an ostrich IS a
> bird - but it just won't succeed.
> 
> ChrisA

I would think Ostrich.fly() should raise NotImplementedError.  Whether 
this violates LSP or not, depends on how you define Bird.

class Bird:
    def fly(self):
        """Commence aviation.

           Note: not all birds can fly.  Calling fly() on
           a flightless bird will raise NotImplementedError.

        """

class Ostrich(Bird):
    def fly(self):
       raise NotImplementedError("ostriches can't fly")

class Sheep(Bird):
    def fly(self):
        self.plummet()