The trouble is that, in good OO programming, we don't make class hierarchies in order to satisfy our inner Linnaeus. We make class hierarchies in order to simplify the code by allowing different parts of it to be changed independently of each other, and to eliminate duplication (which comes to the same thing). Without any context as to what the code needs to accomplish, you can't make a judgment about whether a particular design decision is good or bad.

Here are some real-world examples (drawn from my own code, sorry):

• A Timer is a horizontal strip on the screen with a stripe racing across it. A NumericHalo is a spreading ripple on the screen that fades. A Sound is a thing on the screen that makes a sound when a Timer's stripe hits it. A Trash is a thing that deletes Sounds when you drop them on it. They all inherit from Visible, which represents things that can be drawn on the screen and perhaps respond to mouse clicks or drop events, but they do different things in those three cases. In addition, the Trash and Sounds are subclasses of ImageDisplay, because the way they handle being drawn is simply to display a static image, so that code is factored into a superclass. http://www.canonical.org/~kragen/sw/pygmusic/pygmusic.html#V...

The next two are only reasonable examples if you know the λ-calculus:

• Var, App, and Ind are three kinds of Nodes representing an expression graph: variables, applications, and indirection nodes. But in evaluating an expression using combinator graph reduction, sometimes you need to mutate one kind of Node into another. So instead of making them subclasses of Node, there are Strategy objects called Var_type, App_type, and Ind_type, and Node objects delegate almost all of their behavior to that Strategy object: http://lists.canonical.org/pipermail/kragen-hacks/2006-Novem... Note that this allows the code to later extend the combinator graph type with λ abstractions by adding another Strategy object.

• Doing combinator-graph reduction directly on λ-expressions, you have three classes of graph nodes — variable references, λ abstractions, and function applications. All of these types support a common "graph node protocol", which recursively generates printable representations of them in various formats, such as Scheme, TeX, plain ASCII text, or Perl. In a language like Java, this would be an interface, but in Perl, protocols don't have an explicit representation, so these classes don't inherit from anything in common. These classes implement an additional protocol used for β-reduction, which consists of two methods, "occurs_free" and "substitute". However, β-reduction to normal form requires identifying β-redexes, which is not something that a single expression graph node can do on its own, so this is done by functions that violate encapsulation by making explicit is-a checks. http://lists.canonical.org/pipermail/kragen-hacks/1999-Septe...

The next example is only good if you already know basic algebra:

• The Formula class overloads a bunch of operators to make it convenient to combine formulas into other formulas, and it has subclasses Binop, Unop, Variable, and Constant that represent particular kinds of formulas. These subclasses implement methods derivative (you know, taking the derivative, calculus), identical, simplified, and eval, as well as coercion to strings. Derivatives and simplification in particular are wildly different for different kinds of binary operations, so you could imagine making subclasses of Binop for the five binary operations supported, and that would probably be better than what I in fact did, which was to use dictionaries of lambdas for evaluation, derivatives, and simplification. This would involve replacing instantiations of the Binop class with calls to a factory function. http://lists.canonical.org/pipermail/kragen-hacks/2001-Janua...

• The Subfile class implements most of the same interface as Python's built-in file type, but provides read-only access to only a small byte-range of a file. This means you can pass a Subfile object to a function that expects to be passed a file object, and it will work on that small part of the file. This allows the `_words` function (search for `def _words`) to be written as if it were reading an entire file, separating out the tasks of checking for the end of the byte range and breaking a file into words into two separate objects. http://lists.canonical.org/pipermail/kragen-hacks/2006-Augus...

• UI interacts with the user of a chat client; icb interacts with a chat server; httpreq interacts with a web server. All three of them implement the protocol that asyncore expects of "channel" objects, which involves methods like readable(), handle_read(), writable(), handle_write(), and so on — basically things that manage event-loop I/O on a file descriptor. They also all inherit from asyncore's `dispatcher_with_send` class, which provides default implementations of some of these methods. Two of them also inherit from an `upgradable` class, which provides an implementation of dynamic code upgrade — replacing their current code with code newly loaded from the filesystem. (This allows you to hack the code of the client without closing and reopening your chat connections.) http://lists.canonical.org/pipermail/kragen-hacks/2002-Augus...

(Relevant drive-by dismissal: http://www.netalive.org/swsu/archives/2005/10/in_defense_of_... "this article [by Fowler] finally made me understand what OOP was all about. This is not true for many other articles and yes, I'm looking at you, shitty Car extends Vehicle OOP tutorial.")

The problem with the "Duck extends Bird" kind of example is that it gives you no understanding of the kind of considerations you need to think about in order to decide whether the design decisions discussed above are good or bad. In fact, it actively sabotages that understanding. You can't add code to ducks; you can't refactor ducks; ducks don't implement protocols; you can't create a new species in order to separate some concerns (e.g. file I/O and word splitting); you can't fake the ability to turn a duck into a penguin by moving its duckness into an animal of some other species that can be replaced at runtime; you can't indirect the creation of ducks through a factory that produces birds of several species, and even if you could, the analogy doesn't help at all in understanding why the analogous thing might be a good idea in the Binop case; penguins don't implement the "fly" method that can be found in birds; whether you consider ducks to be birds or simply chordates does not affect the internal complexity of ducks; and you don't go around causing things to fly without knowing what kind of bird they are. (Ducks themselves decide when they want to fly, and they certainly seem to know they're ducks and not vultures.)

So I disagree that the analogy "makes it easier to grok what polymorphism is about". It's misleading; it obscures the relevant while confusing people with the irrelevant.