pandas dataframe的合并（append, merge, concat）

>>> df1 = pd.DataFrame(np.ones((4, 4))*1, columns=list('DCBA'), index=list('4321'))
>>> df2 = pd.DataFrame(np.ones((4, 4))*2, columns=list('FEDC'), index=list('6543'))
>>> df3 = pd.DataFrame(np.ones((4, 4))*3, columns=list('FEBA'), index=list('6521'))
>>> df1
    D    C    B    A
4  1.0  1.0  1.0  1.0
3  1.0  1.0  1.0  1.0
2  1.0  1.0  1.0  1.0
1  1.0  1.0  1.0  1.0
>>> df2
    F    E    D    C
6  2.0  2.0  2.0  2.0
5  2.0  2.0  2.0  2.0
4  2.0  2.0  2.0  2.0
3  2.0  2.0  2.0  2.0
>>> df3
    F    E    B    A
6  3.0  3.0  3.0  3.0
5  3.0  3.0  3.0  3.0
2  3.0  3.0  3.0  3.0
1  3.0  3.0  3.0  3.0
>>> df1 = pd.D