If Python is an extremely popular programming language people like to write about using (it is), you'll expect a lot of people to state that Python is their most proficient language and for that to make its way into the training data. When you ask the LLM, you'll get Python as an answer some fraction of the time and other languages some other fraction.

Those answers are orthogonal to how good the LLM actually is though. In the training data, you'd like a sufficiently robust sample of code (which Python should have) without most of the code being terrible (which Python doesn't have in the wild).

Mind you, IME they do better with Python than very new languages, and they do better with Python tooling than whatever maven build garbage you might have to deal with, but that's faint praise.

Unit tests are a major win for LLMs.

It seems to understand the way you want your unit tests written. So if you have a particular style, it's best to write one or two tests in your style, then it will use that same style when it starts writing tests.

You can do prompts like "I need unit tests for the exceptions that are raised in the SuchAndSuch.function()", and it will do it -- particularly if you have a unit test already written similar to what you'd like.