pandas left join and update existing column

Question

I am new to pandas and can\'t seem to get this to work with merge function:

>>> left       >>> right
   a  b   c       a  c   d 
0  1  4            


        

Answer 1

One way to do this is to set the a column as the index and update:

In [11]: left_a = left.set_index('a')

In [12]: right_a = right.set_index('a')

Note: update only does a left join (not merges), so as well as set_index you also need to include the additional columns not present in left_a.

In [13]: res = left_a.reindex(columns=left_a.columns.union(right_a.columns))

In [14]: res.update(right_a)

In [15]: res.reset_index(inplace=True)

In [16]: res
Out[16]:
   a   b   c   d
0  1   4   7  13
1  2   5   8  14
2  3   6   9  15
3  4   7  12 NaN