Training seems fine. I learn how to write something by looking at example code, then write my own program, that's widely accepted to be a fair use of the code. Same if I learn multiple things from reading encyclopedias, then write an essay, that's good.

However if I memorise that code and write it down that's not fair use. If I copy the encyclopedia that's bad.

The problem then comes into "how trivial can a line be before it's copyrighted"

    def main():
      print("This is copyrighted")
    main()

This is a problem in general, not just in written words. See the recent Ed Sheeran case - https://www.bbc.co.uk/news/articles/cgmw7zlvl4eo