astrein: silence file count in Bazel aspect runs

The "Checking N python files" stderr message caused an
"INFO: From Linting" block per target in CI logs. Print it
only when no --output file is given (interactive use).

Change-Id: I3eafed0b2e9ae0acbee05cdac66bd7b254ed4299

Author: LarsMichelsen

packages/cmk-astrein/cmk/astrein/cli.py

@@ -70,6 +70,11 @@ def main() -> int:
         sys.stderr.write(f"{e}\n")
         return 1
 
+    # Keep interactive runs informative, but stay quiet when writing to an
+    # output file (Bazel aspect), where this would spam the build log.
+    if args.output is None:
+        sys.stderr.write(f"Checking {len(files_to_check)} python files\n")
+
     return _handle_results(
         _run_checkers(files_to_check, workspace_dir, factories),
         args.format,
@@ -135,10 +140,7 @@ def _collect_files(paths: Sequence[Path], workspace_dir: Path) -> list[Path]:
         else:
             raise ValueError(f"Error: Not a file or directory: {path}")
 
-    files_to_check = sorted(set(files_to_check))
-    sys.stderr.write(f"Checking {len(files_to_check)} python files\n")
-
-    return files_to_check
+    return sorted(set(files_to_check))
 
 
 @dataclass