speed up batch black processing

Author: iscai-msft

packages/http-client-python/eng/scripts/setup/run_batch.py

@@ -174,11 +174,15 @@ def update_progress():
     # Run black formatting after all codegen completes. Running black separately
     # avoids duplicating black's import/startup cost in each worker process.
     if output_dirs:
-        from pygen.black import BlackScriptPlugin
+        import subprocess
 
         print(f"Formatting {len(output_dirs)} packages with black...")
-        for d in output_dirs:
-            BlackScriptPlugin(output_folder=d).process()
+        # Use black CLI with all directories at once — black handles parallelism internally.
+        # This is much faster than calling format_file_contents() per-file sequentially.
+        subprocess.run(
+            [sys.executable, "-m", "black", "--line-length", "120", "--fast", "--quiet"] + output_dirs,
+            check=True,
+        )
 
 
 if __name__ == "__main__":