Architecture Patterns with Python

Chapter 13. Dependency Injection (and Bootstrapping)

Dependency injection (DI) is regarded with suspicion in the Python world. And we’ve managed just fine without it so far in the example code for this book!

In this chapter, we’ll explore some of the pain points in our code that lead us to consider using DI, and we’ll present some options for how to do it, leaving it to you to pick which you think is most Pythonic.

We’ll also add a new component to our architecture called bootstrap.py; it will be in charge of dependency injection, as well as some other initialization stuff that we often need. We’ll explain why this sort of thing is called a composition root in OO languages, and why bootstrap script is just fine for our purposes.

Figure 13-1 shows what our app looks like without a bootstrapper: the entrypoints do a lot of initialization and passing around of our main dependency, the UoW.

Tip

If you haven’t already, it’s worth reading Chapter 3 before continuing with this chapter, particularly the discussion of functional versus object-oriented dependency management.

Figure 13-1. Without bootstrap: entrypoints do a lot

Tip

The code for this chapter is in the chapter_13_dependency_injection branch on GitHub:

git clone https://github.com/cosmicpython/code.git cd code git checkout chapter_13_dependency_injection # or to code along, checkout the previous chapter: git checkout chapter_12_cqrs ...