Looking for a list subclassing example...

[Kragen Sitaker]

Erik Max Francis <max at alcyone.com> writes:
> Kragen Sitaker wrote:
> > Why do you think it's a good idea to subclass 'list'?
> > 
> > I don't have one to show, but I think it's generally not a good idea.
> 
> Why do you think it was made possible in 2.2, then?

As an experiment.  In October, Guido said:

    Subclassing list and dictionary etc. should be seen as an experimental
    feature; I don't want to fully fix the semantics yet in all cases.
     . . .
    Subclassing a built-in type is appropriate when either (a) you want to
    use it in a context where a genuine list/dictionary/file/etc.; or (b)
    you want the speed advantage of the built-in type.  In both cases you
    have to live with some restrictions.  Remapping the fundamental
    accessors (like __getitem__) is probably not a good idea in either
    case.  Adding new state and behavior is fine.

[from
<http://groups.google.com/groups?selm=mailman.1004342434.4625.python-list%40python.org>]

I'm tempted to be flippant and say "to confuse new users" or "as a
sick joke", but as Guido points out, there are at least two valid
reasons to do it: to optimize your code and to work around breakage
--- well, let's say inconsistent, inflexible behavior --- caused by
other people's optimizations.