Can you use variables in

BUILD

files? Or some other wildcard substitution mechanism?

BUILD files are just starlark so you can define lists and dictionaries and such outside of target definitions.

I think

BUILD

files are normal Python (with restrictions on imports). You can share also things across multiple

BUIlD

files via macros: https://www.pantsbuild.org/docs/macros

you are correct! I was thinking of bazel which uses starlark

Yup, Pants 2 doesn't use Starlark. You've got Python itself at your disposal.