pandas向同一个csv多次写入

with open('a', 'w', encoding='utf-8') as f:

        pd.DataFrame(columns=['f']).to_csv(f, index=False)

        df1 = pd.DataFrame([1, 2, 3], columns=['f'])
        df2 = pd.DataFrame([4, 5, 6], columns=['f'])

        df1.to_csv(f, mode='a+', index=False,  header=False)
        df2.to_csv(f, mode='a+', index=False,  header=False)

df = pd.read_csv('a')
print(df)