Python

Read other people's code... particularly code by experienced developers. One good way to do that is to single-step debugging through the test code in a well-known package, stepping into the code being tested.

I suppose if you don't know how test frameworks like pytest work, tackling how they work and how to do single-stepping with some toy example code will be a prerequisite for the above, as will spending some time studying how packages are made. (The latter may seem unattractively tedious, but the knowledge will pay off even if you never become an expert at making your own packages.)

These exercises are very likely to expose weaknesses in your understanding of all sorts of things. Be patient and keep studying!