#20576 2.20 runs ruff and yapf on BUILD files without opting in to them Issue created by huonw Describe the bug The default behaviour of

pants update-build-files ::

seems to have changed in 2.20, compared to 2.19. In particular, it seems to now build and run yapf and ruff, in addition to black, without opting in to the ruff or yapf formatters via the

pants.backend.build_files.fmt.*

backends. (https://www.pantsbuild.org/2.20/docs/using-pants/key-concepts/backends#available-stable-backends) Reproducer:

cd $(mktemp -d)

cat > pants.toml <<EOF
[GLOBAL]
pants_version = "2.20.0a0"
backend_packages = []
EOF

# https://docs.astral.sh/ruff/formatter/black/#single-element-tuples-are-always-parenthesized
echo '(1, 2),' > BUILD

pants update-build-files ::

This sets up a build file that has different expected formatting for black vs. ruff by abuse a difference between them. Running with

PANTS_VERSION=2.19.0 bash run.sh

(or anything up to

PANTS_VERSION=2.20.0.dev7 bash run.sh

) has output:

12:37:40.96 [INFO] Initializing scheduler...
12:37:41.14 [INFO] Scheduler initialized.
12:37:45.89 [INFO] Starting: Building black.pex from <resource://pants.backend.python.lint.black/black.lock>
12:37:46.76 [INFO] Completed: Building black.pex from <resource://pants.backend.python.lint.black/black.lock>
12:37:47.37 [INFO] No required changes to BUILD files found.However, there may still be deprecations that `update-build-files` doesn't know how to fix. See <https://www.pantsbuild.org/v2.19/docs/upgrade-tips> for upgrade tips.

Notably: • builds black ✅ • doesn't reformat ✅ Running with

bash run.sh

has output:

PANTS_LOCAL_CACHE=false  bash run.sh
12:39:35.21 [INFO] Initializing scheduler...
12:39:35.39 [INFO] Scheduler initialized.
...

12:39:37.04 [INFO] Starting: Building black.pex from <resource://pants.backend.python.lint.black/black.lock>
12:39:37.82 [INFO] Completed: Building black.pex from <resource://pants.backend.python.lint.black/black.lock>
12:39:38.11 [INFO] Starting: Building yapf.pex from <resource://pants.backend.python.lint.yapf/yapf.lock>
12:39:38.86 [INFO] Completed: Building yapf.pex from <resource://pants.backend.python.lint.yapf/yapf.lock>
12:39:38.97 [INFO] Starting: Building ruff.pex from <resource://pants.backend.python.lint.ruff/ruff.lock>
12:39:39.88 [INFO] Completed: Building ruff.pex from <resource://pants.backend.python.lint.ruff/ruff.lock>
Updated BUILD:
  - Format with Ruff

Notably: • builds black ✅ and ruff ❌ and yapf ❌ • reformats with ruff ❌ Pants version 2.20.0a0 This seems to be a regression between that and 2.20.0.dev7 likely related to #20411. • https://github.com/pantsbuild/pants/releases/tag/release_2.20.0a0 • release_2.20.0.dev7...release_2.20.0a0 OS macOS Additional info N/A pantsbuild/pants