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copied!<p>Inheritance defines an "Is-A" relationship.</p> <pre><code>class Point( object ): # some set of features: attributes, methods, etc. class PointWithMass( Point ): # An additional feature: mass. </code></pre> <p>Above, I've used inheritance to formally declare that PointWithMass is a Point.</p> <p>There are several ways to handle object <code>P1</code> being a PointWithMass as well as Point. Here are two.</p> <ol> <li><p>Have a reference from <code>PointWithMass</code> object <code>p1</code> to some Point object <code>p1-friend</code>. The <code>p1-friend</code> has the <code>Point</code> attributes. When <code>p1</code> needs to engage in <code>Point</code>-like behavior, it needs to delegate the work to its friend.</p></li> <li><p>Rely on language inheritance to assure that all features of <code>Point</code> are also applicable to my <code>PointWithMass</code> object, <code>p1</code>. When <code>p1</code> needs to engage in <code>Point</code>-like behavior, it already <em>is</em> a <code>Point</code> object and can just do what needs to be done.</p></li> </ol> <p>I'd rather not manage the extra objects floating around to assure that all superclass features are part of a subclass object. I'd rather have inheritance to be sure that each subclass is an instance of it's own class, plus is an instance of all superclasses, too.</p> <p>Edit.</p> <p>For statically-typed languages, there's a bonus. When I rely on the language to handle this, a <code>PointWithMass</code> can be used anywhere a <code>Point</code> was expected.</p> <p>For really obscure abuse of inheritance, read about C++'s strange "composition through private inheritance" quagmire. See <a href="https://stackoverflow.com/questions/255447/any-sensible-examples-of-creating-inheritance-without-creating-subtyping-relati">Any sensible examples of creating inheritance without creating subtyping relations?</a> for some further discussion on this. It conflates inheritance and composition; it doesn't seem to add clarity or precision to the resulting code; it only applies to C++.</p>

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