report_path(base, year, month, name) that joins the pieces into base/<year>/<month>/<name>.csv using os.path.join (month zero-padded to 2 digits), letting the OS pick the separator.report_path('data', 2026, 7, 'sales')'data/2026/07/sales.csv' (or backslashes on Windows)
join inserts the platform separator.
str(year) and f"{month:02d}" prepare the parts.
Append .csv to the name before joining.