Week 8 SLOG

During the reading week, I stumbled upon metaclasses while looking for some ideas/inspiration for the second assignment. What that began as a misunderstanding (as always) of the requirements of the first part of a2 (make public attributes read-only) became a really (valuable?) learning experience when I found out that not only are all classes in Python objects, they are instances of metaclasses, the most common being the metaclass type. type(), for the uninitiated actually has 2 functions.

The first, of course, is to return the data type (note that since Python does not have data types in the traditional sense, the term does not necessarily apply) of an object. To do that, only one argument, the object itself has to be passed to type().

What most Python users don't know is that type can alternately act as a class 'factory', accepting three arguments, and returning a class (which is actually an object itself). The first is a str with the desired name of the class to be returned, the second is a tuple of the classes to inherit from, and the third is a dict of the desired class attributes. Note that class attributes are the variables that can be accessed by calling <class name>.<variable name>, and the embedded methods in the class.

So what exactly do metaclasses do? Well, when used outside of class definitions, it just returns a class from the arguments passed. The interesting part is when a metaclass argument is passed to class defintions. To do so, Python users can either write a function (or class method - very complicated) which returns a class and then add the argument metaclass = <desired metaclass> to the class defintion. When the class is evaluated prior to code execution, the metaclass will override the __new__ method in the class (__new__ is the default metaclass, which returns a basic class object), thus modifying the class before instances of the new class can be created.

Of course, that is not all Python users can do with metaclasses. Should the metaclass be defined as a class itself, there are many other things that can be done. But as I said, doing that is extremely complicated (no kidding!) and I'm not necessarily confident enough to write it here yet.