Pandas Merge Many Rows of One Dataframe into Fewer Rows of Second Dataframe

Question

Is there an easy way to merge two dataframes such that df2 adds all its rows with matching ‘on’ values as new columns in df1? Open to other methods of joining the data as well.

e.g.
Matching on Course Offering Code and User Id

df1:

Course Offering Code	User Id
001	1
001	2

df2:

Course Offering Code	User Id	Assignment	grade%
001	1	A01	65
001	1	A02	85
001	1	A03	95
001	1	A04	64
001	2	A01	87
001	2	A02	86
001	2	A03	82
001	2	A04	90

I had tried pd.merge(df1, df2, on=['User Id', 'Course Offering Code']) and was hoping for the following:

desired_df

Course Offering Code	User Id	Assignment_x	grade%_x	Assignment_y	grade%_y	Assignment_z	grade%_z	Assignment_a	grade%_a
001	1	A01	65	A02	85	A03	95	A04	64
001	2	A01	87	A02	86	A03	82	A04	90

Asked By: Kevin

||

Source

Answer 1

You can do this one with pandas.DataFrame.pivot :

def flatten_cols(df):
    df.columns = ['_'.join(map(str, x)) for x in df.columns]
    df = df[sorted(df.columns, key=lambda x: int(x.split("_")[-1]))]
    return df

out = (
        df1
          .merge(df2, on=["Course Offering Code", "User Id"], how="left")
          .assign(idx=lambda x: x.groupby(['Course Offering Code', 'User Id']).cumcount()+1)
          .pivot(index= ["Course Offering Code", "User Id"],
                 columns= "idx", values=["Assignment", "grade%"])
          .pipe(flatten_cols)
          .reset_index()
      )

# Output :

print(out.to_string())

  Course Offering Code  User Id Assignment_1 grade%_1 Assignment_2 grade%_2 Assignment_3 grade%_3 Assignment_4 grade%_4
0                  001        1          A01       65          A02       85          A03       95          A04       64
1                  001        2          A01       87          A02       86          A03       82          A04       90

Answered By: abokey

Pandas Merge Many Rows of One Dataframe into Fewer Rows of Second Dataframe

Question:

Answers:

# Output :

Course Offering Code	User Id	Assignment	grade%
001	1	A01	65
001	1	A02	85
001	1	A03	95
001	1	A04	64
001	2	A01	87
001	2	A02	86
001	2	A03	82
001	2	A04	90

Course Offering Code	User Id	Assignment	grade%
001	1	A01	65
001	1	A02	85
001	1	A03	95
001	1	A04	64
001	2	A01	87
001	2	A02	86
001	2	A03	82
001	2	A04	90

Course Offering Code	User Id	Assignment	grade%
001	1	A01	65
001	1	A02	85
001	1	A03	95
001	1	A04	64
001	2	A01	87
001	2	A02	86
001	2	A03	82
001	2	A04	90