Stylistic question about inheritance

[Guy Bolton King]

"Andrew Koenig" <ark at acm.org> writes:

> "Lonnie Princehouse" <finite.automaton at gmail.com> wrote in message 
> news:1112300578.456411.274110 at f14g2000cwb.googlegroups.com...
> 
> > If you try this sort of inheritance, I'd recommend writing down the
> > formal grammar before you start writing classes.  Don't try to define
> > the grammar through the inheritance hierarchy; it's too easy to
> > accidentally build a hierarchy that can't be translated into a
> > single-pass-parsable grammar...
> 
> Understood.  I was using expression trees as a contrived example, and really 
> want to know about the Python community's stylistic preferences for defing 
> such hierarchies that don't absolutely need a root.

Oddly enough, I've just been pondering the same question (albeit for
Perl, but the same reasoning applies).  The only cases I've found
useful thus far are:

  - implementation inheritance (in the case of default methods in a
    callback interface class):

      class CallbackInterface:

          def handleEvent(self, event):
              """Handle an event"""
              pass

          def handleSignal(self, signal):
              """Handle a signal"""
              pass

    This also helps to document what's expected of callback classes,
    even though they don't _have_ to inherit CallbackInterface
    (enforcing this through isinstance() in the calling class would be
    rude).

  - hierarchies of exception classes (allowing one to catch general
    classes of exceptions, since except implicitly uses isinstance(),
    rather than a specific class).  Of course, Python already has a
    hierarchy of exceptions.  I had to implement my own for Perl.

>From a brief skim of http://www.python.org/moin/PythonThreeDotOh it
looks like interfaces _may_ be added to Python 3.0, but they sound
more like (IIRC) ML's signatures and C++'s Standard Library
requirements i.e. a requirement that the class implements certain
functions, rather than a requirement to inherit from a particular base
class.

Guy.