In [1]: pin1=  pd.Series(np.random.randn(2), index=['2', '4'])
In [2]: pin2= pd.Series(np.random.randn(3), index=['0', '1', '7'])
In [3]: pin3=pd.merge(pin1,pin2,how='outer')
In [4]: pin3
Out [4]:
0    0.2941
1    0.2869
2    1.7098
3   -0.2126
4    0.2696

Out [4]:
 0    0.2941
 1    0.2869
 2    1.7098
 4   -0.2126
 7    0.2696

pd.concat([pin1, pin2]).sort_index()

In [3732]: pin1.combine_first(pin2)
Out[3732]:
0   -0.820341
1    0.492719
2   -0.785723
4   -1.815021
7    2.027267
dtype: float64

In [3734]: pin1.append(pin2).sort_index()
Out[3734]:
0   -0.820341
1    0.492719
2   -0.785723
4   -1.815021
7    2.027267
dtype: float64

In [3735]: pin1
Out[3735]:
2   -0.785723
4   -1.815021
dtype: float64

In [3736]: pin2
Out[3736]:
0   -0.820341
1    0.492719
7    2.027267
dtype: float64

pin1.align(pin2,join='outer')[0].fillna(pin1.align(pin2,join='outer')[1])
Out[991]:
0   -0.278627
1    0.009388
2   -0.655377
4    0.564739
7    0.793576
dtype: float64