Testing
I have done functional and unit testing. There was even a period in my career when testing was my main responsibility, but this was some time ago.
Ideally, you want to test everything. But there is a lot of prototyping and experiments, especially in Python, and writing tests for throwaway code looks like a waste of time. And there are deadlines, and testing often is the first casualty of deadlines. One place where testing is rigorously enforced is Open Source development, and it is definitely the case with Pandas. Any Pandas contributions should have a corresponding test case, and I learned a lot about Python testing as a result of my Pandas contributions .
In C++ I wrote a client/server Test Bed for System Testing of infrastructure. It was the time when testing was my main responsibility.
In Java and Python, I wrote a fair amount of unit tests, but they were mostly run of the mill unit tests, similar to what you can find online.
Below are some of the libraries and tools that I was using:
pytest, unittest, mocking, Selenium. JUnit, Mockito, PowerMock