Pandas: Concatenate dataframe and keep duplicate indices

Question

I have two dataframes that I would like to concatenate column-wise (axis=1) with an inner join. One of the dataframes has some duplicate indices, but the rows are not duplicates

Answer 1

You can perform a merge and set the params to use the index from the lhs and rhs:

In [4]:    
df1.merge(df2, left_index=True, right_index=True)
Out[4]:
   b  c
a      
1  2  5
1  3  5
2  4  6

[3 rows x 2 columns]

Concat should've worked, it worked for me:

In [5]:

pd.concat([df1,df2], join='inner', axis=1)
Out[5]:
   b  c
a      
1  2  5
1  3  5
2  4  6

[3 rows x 2 columns]