Adding a new line after appending to a data frame

Question

I need to add a new line after appending each time to a data frame. Here is my code:

import os
import pandas as pd
path = os.getcwd()
files = os.listdir(path)
files_xls = [f for f in files if f[-3:] == 'lsx']
df = pd.DataFrame()
for f in files_xls:
    data = pd.read_excel(f)
    df = df.append(data)