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[Python-ideas] OrderedCounter and OrderedDefaultDict

Greg Ewing greg.ewing at canterbury.ac.nz
Thu Oct 22 00:30:03 CEST 2015 
Carl Meyer wrote:
> It takes her a couple minutes more to get to the
> real point, which starts at the slide "inheritance is for
> specialization, not for sharing code."

I'm not sure there's any content in that statement.
In a duck-typed language, sharing code is the *only*
reason to use inheritance.

Also, what is "specialisation" anyway? Any time
you have two objects with the same interface, and
mostly-similar but slightly different behaviours,
you could regard one as being a specialisation of
the other. Whether they share any code or not is
an implementation detail.

I think the real issue is that if you specialise
something by inheriting and overriding random
methods, you make your class fragile with respect
to changes in the base class. If the author of
the base class changes something undocumented about
the way its methods interact with each other, your
class may break.

With pluggable behaviours, there is a well-defined
interface between the main class and the classes
representing the behaviours, so that sort of thing
is much less likely to happen.

You could get the same result with inheritance by
designating some methods as "designed to be
overridden", and documenting how they interact with
the rest of the class. Effectively you are then
defining an interface *within* the class and
between the class and its subclasses. This is
really just another implementation of pluggable
behaviours where the plugging mechanism consists
of overriding one or more methods.

So to summarise, I would say that the issue here
isn't really composition vs. inheritance, but
structured vs. ad-hoc behaviour modification.

I wouldn't say that one is always better than the
other. Sometimes you need to do things that the
author of a class didn't anticipate, and then the
ability to override things in an ad-hoc manner is
very useful. But I think the speaker is right to
point out that doing *everything* that way can
lead to a lot of trouble.

-- 
Greg
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