In [1]: import pandas as pd In [6]: left =pd.DataFrame({ 'A':['A0','A1','A2','A3'], 'B':['B0','B1','B2','B3'], 'key':['K0','K1','K2','K3'],}) right =pd.DataFrame({ 'C':['C0','C1','C2','C3'], 'D':['D0','D1','D2','D3'], 'key':['K0','K1','K2','K3'],}) I