多个dataframe如何to_excel不同的sheet里

import pandas as pd

df1 = pd.DataFrame({'a':[1,2,3], 'b':[4,5,6]}, columns=['a', 'b'])
df2 = pd.DataFrame({'a':[7,8,9], 'b':[6,5,4]}, columns=['a', 'b'])
# print(df1)
writer = pd.ExcelWriter('a.xls')
df1.to_excel(writer, sheet_name='sheet1', index=False)
df2.to_excel(writer, sheet_name='sheet2', index=False)
writer.save()