Chapter 11. Interfaces: From Protocols to ABCs

An abstract class represents an interface.¹

Bjarne Stroustrup, Creator of C++

Interfaces are the subject of this chapter: from the dynamic protocols that are the hallmark of duck typing to abstract base classes (ABCs) that make interfaces explicit and verify implementations for conformance.

If you have a Java, C#, or similar background, the novelty here is in the informal protocols of duck typing. But for the long-time Pythonista or Rubyist, that is the “normal” way of thinking about interfaces, and the news is the formality and type-checking of ABCs. The language was 15 years old when ABCs were introduced in Python 2.6.

We’ll start the chapter by reviewing how the Python community traditionally understood interfaces as somewhat loose—in the sense that a partially implemented interface is often acceptable. We’ll make that clear through a couple examples that highlight the dynamic nature of duck typing.

Then, a guest essay by Alex Martelli will introduce ABCs and give name to a new trend in Python programming. The rest of the chapter will be devoted to ABCs, starting with their common use as superclasses when you need to implement an interface. We’ll then see when an ABC checks concrete subclasses for conformance to the interface it defines, and how a registration mechanism lets developers declare that a class implements an interface without subclassing. Finally, we’ll see how an ABC can be programmed to automatically “recognize” ...