import os

class Testlist_files:
    def test_list_files_1(self, tmp_path, capsys):
        d = tmp_path / "sub"
        d.mkdir()
        p = d / "hello.txt"
        p.write_text("content")
        list_files(tmp_path)
        captured = capsys.readouterr()
        assert captured.out == "hello.txt\n"

def list_files(path):
    if not os.path.exists(path):
        print("Invalid directory path.")
        return
    
    for root, dirs, files in os.walk(path):
        for file in files:
            print(file)