Run pre-commit on PR changed files only

Switch workflow to run pre-commit only for pull requests and only on files changed in the PR. Adds a detect_changes job to fetch the base ref and output changed file list, then a conditional precommit job that runs only when there are changed files. Updates checkout and setup actions (actions/checkout@v4, setup-python@v5), bumps Python to 3.12, installs pre-commit, and runs pre-commit in a CI check-only mode on the changed files to speed up CI and avoid running on the whole repo.

Author: C-Achard

.github/workflows/format.yml

@@ -1,23 +1,61 @@
-name: pre-commit-format
+name: pre-commit (PR only on changed files)
 
 on:
-  push:
-    branches: [main]
   pull_request:
-    branches: [main]
+    types: [opened, synchronize, reopened]
 
 jobs:
-  pre_commit_checks:
+  detect_changes:
     runs-on: ubuntu-latest
+    outputs:
+      changed: ${{ steps.changed_files.outputs.changed }}
+
     steps:
-      - uses: actions/checkout@v6
+      - name: Checkout full history
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Detect changed files
+        id: changed_files
+        run: |
+          git fetch origin ${{ github.base_ref }}
+          CHANGED_FILES=$(git diff --name-only origin/${{ github.base_ref }}...HEAD)
+
+          {
+            echo "changed<<EOF"
+            echo "$CHANGED_FILES"
+            echo "EOF"
+          } >> "$GITHUB_OUTPUT"
+
+      - name: Show changed files
+        run: |
+          echo "Changed files:"
+          echo "${{ steps.changed_files.outputs.changed }}"
+
+  precommit:
+    needs: detect_changes
+    runs-on: ubuntu-latest
+    if: ${{ needs.detect_changes.outputs.changed != '' }}
+
+    steps:
+      - name: Checkout PR branch
+        uses: actions/checkout@v4
         with:
           fetch-depth: 0
           ref: ${{ github.head_ref }}
 
-      - uses: actions/setup-python@v6
+      - name: Set up Python
+        uses: actions/setup-python@v5
         with:
-          python-version: '3.10'
+          python-version: "3.12"
+
+      - name: Install pre-commit
+        run: pip install pre-commit
 
-      - run: pip install pre-commit
-      - run: pre-commit run --all-files
+      - name: Run pre-commit (CI check-only stage) on changed files
+        env:
+          CHANGED_FILES: ${{ needs.detect_changes.outputs.changed }}
+        run: |
+          mapfile -t files <<< "$CHANGED_FILES"
+          pre-commit run --hook-stage manual --files "${files[@]}" --show-diff-on-failure