Try A_D's "diff only" flake8 checking

.github/workflows/python-app.yml

@@ -26,8 +26,25 @@ jobs:
         pip install flake8 pytest
         if [ -f requirements-dev.txt ]; then pip install -r requirements-dev.txt; fi
     - name: Lint with flake8
+      env:
+        ROOT_SHA: ${{github.base_ref}}
+        
       run: |
-        # stop the build if there are Python syntax errors or undefined names
-        flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
+        DATA=$(python3 <<EOF
+        import json
+        import os
+        with open(os.getenv('GITHUB_EVENT_PATH')) as f:
+            data = json.load(f)
+        
+        typ = os.getenv('GITHUB_EVENT_NAME')
+        
+        if 'pull_request' in typ:
+            print(data['base']['ref'])
+        elif 'push' in typ:
+            print(data['before'])
+        EOF
+        )
+        # stop the build if there are Python syntax errors or undefined names, ignore existing 
+        git diff "$DATA" | flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics --diff
         # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
-        flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
+        git diff "$DATA" | flake8 . --count --exit-zero --statistics --diff