Source: 📖 Test Driven Development with Python ch23 p401

Don't work on lower level code with failing tests in the upper layers

You should not start working on lower-level (or 'inner') implementations if you have failing tests in the upper layers of code — for example, you should not work on your Django model code if you have tests failing for the views or even the URL layer.

If you have cross-layer dependencies, it means that your tests need to be more isolated, they need to test more self-contained problems. This usually means breaking them down into multiple smaller tests, and using mocks if necessary.

See also:

• Outside-In testing