Adding a combination in a datafra, which is missing. Pandas data frame

Question

Problem: I have the following dataframe (this is a part of a large data frame)

        Name X      Name Y       Value
0       joe         mark         12
1       joe         alice        13
2       joe         bert         14
3       mark        joe          15
4       mark        alice        14
5       alice       joe          16
6       alice       bert         30
7       alice       mark         50

Requested change to the data frame:

        Name X      Name Y       Value
0       joe         joe          0
1       joe         mark         12
2       joe         alice        13
3       joe         bert         14
4       mark        mark         0
5       mark        joe          15
6       mark        alice        14
7       alice       alice        0
8       alice       joe          16
9       alice       bert         30
10       alice       mark        50

Note the order. Reason: I want the pivot table to get the diagonal combination (Joe,Joe) with value 0, (mark, mark) with value 0 etc.

I have looked at pandas documenation, and I do not find the proper way/implement to do it.

Answer 1

Create helper DataFrame by DataFrame.drop_duplicates and DataFrame.assign and add to original DataFrame by concat, last sort output by DataFrame.sort_index:

out = (pd.concat([df.drop_duplicates('Name X')
                    .assign(**{'Name Y': lambda x: x['Name X'], 'Value':0}),
                 df])
         .sort_index(kind='stable', ignore_index=True))
print (out)
   Name X Name Y  Value
0     joe    joe      0
1     joe   mark     12
2     joe  alice     13
3     joe   bert     14
4    mark   mark      0
5    mark    joe     15
6    mark  alice     14
7   alice  alice      0
8   alice    joe     16
9   alice   bert     30
10  alice   mark     50