Clean Code in Python

The dependency flow

Notice that dependencies flow in only one direction, as they move closer to the kernel, where the business rules lie. This can be traced by looking at the import statements. The application imports everything it needs from storage, for example, and in no part is this inverted.

Breaking this rule would create coupling. The way the code is arranged now means that there is a weak dependency between the application and storage. The API is such that we need an object with a get() method, and any storage that wants to connect to the application needs to implement this object according to this specification. The dependencies are therefore inverted—it's up to every storage to implement this interface, in order to create an object ...