pandas中一列拆分成两列

import pandas as pd
df = pd.DataFrame([['a1','a2', 'a3_a4'], ['b4', 'b5', 'b6_b7']],columns=['A','B','C'])
xx = df[['A', 'B']]
print(xx)
xx["C"], xx["D"] =df['C'].str.split('_', expand=True).values.T
print(xx)