Merge items on dataframes with duplicate values

Question

So I have a dataframe (or series) where there are always 4 occurrences of each of column \'A\', like this:

df = pd.DataFrame([[\'foo\'],
                   [         


        

Answer 1

You'll need to create surrogate columns with groupby + cumcount to deduplicate your rows, then include those columns when calling merge:

a = df.assign(D=df.groupby('A').cumcount())
b = df_key.assign(D=df_key.groupby('A').cumcount())

a.merge(b, on=['A', 'D'], how='left').drop('D', 1)

     A    B    C
0  foo  1.0  2.0
1  foo  3.0  4.0
2  foo  NaN  NaN
3  foo  NaN  NaN
4  bar  5.0  9.0
5  bar  2.0  4.0
6  bar  1.0  9.0
7  bar  NaN  NaN