so, what is the rule on "where does one have to have a BUILD file, and what contents are required?"

You need a target to cover every file that you want Pants to know about. A good default is to have one BUILD file per distinct directory: https://www.pantsbuild.org/docs/targets#target-granularity As Danny explained, you can also have coarser targets that cover subdirectories too by using recursive globs like

**/*.py

in the

sources

field