dataframeを列加算、行加算

>>> import pandas as pd
>>> df = pd.DataFrame({"x":['a','b','c','d'],"y":['aa','bb','cc','dd'],"z":['aaa','bbb','ccc','ddd']})
>>> df
   x   y    z
0  a  aa  aaa
1  b  bb  bbb
2  c  cc  ccc
3  d  dd  ddd
【1】          
>>> "_".join(df['x'])
'a_b_c_d'
>>> "".join(df['x'])
'abcd'
【2】          ，        
>>> df['a'] = df['x']+df['y']+df['z']
>>> df
   x   y    z       a
0  a  aa  aaa  aaaaaa
1  b  bb  bbb  bbbbbb
2  c  cc  ccc  cccccc
3  d  dd  ddd  dddddd
>>>