Robust Python

Chapter 11. Defining Your Interfaces

You have learned how to create your own user-defined types, but creating them is just half the battle. Now developers have to actually use your types. To do this, they use your type’s API. This is the set of types and related functions, along with any external functions, that a developer interacts with to use your code.

Once you get your types in front of users, those types will be used (and abused) in ways that you never thought of. And once the developers depend on your types, it will be hard to change their behavior. This gives rise to what I call the Paradox of Code Interfaces:

You have one chance to get your interface right, but you won’t know it’s right until it’s used.

As soon as developers use the types you create, they come to depend on the behavior that those types encompass. If you try to make a backward-incompatible change, you potentially break all calling code. The riskiness in changing your interface is proportional to the amount of outside code depending on it.

This paradox doesn’t apply if you control all the code that depends on your type; you can change it. But as soon as that type hits production, and people start using it, you’ll find it difficult to change. In a large codebase, where robustness and maintainability matter, coordinating the change and buy-in needed to make a sweeping change is costly. It becomes near impossible if your type is used by entities outside your organizational control, such as open ...