python DataFrame join()

 left = pd.DataFrame({'A': ['A0', 'A1', 'A2'],
                     'B': ['B0', 'B1', 'B2']},
                      index=['K0', 'K1', 'K2'])
right = pd.DataFrame({'C': ['C0', 'C2', 'C3'],
                        'D': ['D0', 'D2', 'D3']},
                        index=['K0', 'K2', 'K3'])

join 就是按照 dataframe 的index 对齐

left.join(right)

     A   B    C    D
K0  A0  B0   C0   D0
K1  A1  B1  NaN  NaN
K2  A2  B2   C2   D2