In this section, we'll flirt with the idea of Pretty Poor Polymorphism. Inspection of argument values is a Python programming practice that should be isolated to a few special cases.

Well-done polymorphism follows what is sometimes called the Liskov Substitution Principle. Polymorphic classes can be used interchangeably. Each polymorphic class has the same suite of properties. For more information, visit http://en.wikipedia.org/wiki/Liskov_substitution_principle.

Overusing isinstance() to distinguish between the types of arguments can lead to a needlessly complex (and slow) program. Instance comparisons are made all the time, but errors are generally only introduced through software maintenance. Unit testing is a far better way to find programming errors than verbose type-checking in the code.

Method functions with lots of isinstance() methods can be a symptom of a poor (or incomplete) design of polymorphic classes. Rather than having type-specific processing outside...