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

Here's a way to do it with join:

In [632]: t = left.set_index('a').join(right.set_index('a'), rsuffix='_right')

In [633]: t
Out[633]: 
   b   c  c_right   d
a                    
1  4   9        7  13
2  5  10        8  14
3  6  11        9  15
4  7  12      NaN NaN

Now, we want to set null values of c_right (which is from the right dataframe) with values from c column from the left dataframe. Updated the below process with a method taking from @John Galt's answer

In [657]: t['c_right'] = t['c_right'].fillna(t['c'])

In [658]: t
Out[658]: 
   b   c  c_right   d
a                    
1  4   9        7  13
2  5  10        8  14
3  6  11        9  15
4  7  12       12 NaN

In [659]: t.drop('c_right', axis=1)
Out[659]: 
   b   c   d
a           
1  4   9  13
2  5  10  14
3  6  11  15
4  7  12 NaN