def METHOD_NAME(*, roots=None, python_paths=None):
    """Helper to retrieve paths of Python files."""
    if python_paths is None:
        python_paths = ["setup.py"]
    if roots is None:
        roots = ["charmcraft", "tests"]
    for root in roots:
        for dirpath, dirnames, filenames in os.walk(root):
            for filename in filenames:
                if filename.endswith(".py"):
                    python_paths.append(os.path.join(dirpath, filename))
    return python_paths