Inheritance

To teach you about another testing framework for Python, we must first turn our attention to another OOP concept: inheritance.

In Python, every class is declared as extending some base class (except for object, which has no base class). When you extend a class, your extension (the subclass) inherits all of the methods of the superclass.

To get a feel for why that might be a good thing, let’s think about logging. It’s a common activity to want to record how a program is running. While it’s possible to just use print calls throughout, that ends up being brittle and hard to work with. So it makes sense to have an explicit notion of a ‘logger’. Let’s build a simple one:

Suppose we wanted to take our existing logger and have a timestamp on each log. Inheritance makes that easy: we can make a TimeLogger by extending Logger.

Or maybe we’d rather simply count the number of messages logged. We could make a different subclass of Logger that counted as it ran:

It’s worth remarking on super() in CountingLogger, which says, “now run the method of my super class (i.e., Logger)”. Here CountingLogger increases its count in emit to that it doesn’t ever double count things, i.e., if format were called more than once for each emit, say. Both CountingLogger and TimeLogger inherit Logger.log, i.e., without saying anything, each of these classes had a function log that called the appropriate methods of the subclass. That is, when doit calls logger.log on the TimeLogger, the Logger.log implementation will use TimeLogger.format, because self is TimeLogger. This phenomenon is called dynamic dispatch.

In dynamic dispatch, the receiver of a method determines which method implementation is called. Each time, it starts at the receiver’s actual type, search that type first and then searching through the base classes. The exact details of Python method resolution order are very complex.

Fortunately, it’s not that important to know all of the details (most of the time). Very often, you can figure out what you need about a class in a library by looking at it and its superclasses. Typically, documentation will guide you well here.

Subclasses

You can ask questions about inheritance using the issubclass function. For example:

If issubclass(A, B) and isinstance(x, A), then isinstance(x, B) will be true, also.