Merging two DataFrames with multiple rows for the same key

I have medical data split into two different CSVs, and I need to merge them. One data set contains basic demographic information, and the second contains diagnosis codes. Each patient is assigned a unique identification number called INC_KEY, which I've simplified to simple numbers, as shown in this example:

df1:

INC_KEY   SEX    AGE
1         F      40
2         F      24  
3         M      66

df2:

INC_KEY   DCODE
1         BW241ZZ
1         BW28ZZZ
2         0BH17EZ
3         05H633Z
2         4A103BD
3         BR30ZZZ	
1         BF42ZZZ

I need to merge the two dataframes with the output containing the three rows as seen in df1 with appended columns for each dcode respective to that patient. Like this:

INC_KEY   SEX    AGE   DCODE1     DCODE2     DCODE3
1         F      40    BW241ZZ    BW28ZZZ    BF42ZZZ
2         F      24    0BH17EZ    4A103BD    N/A
3         M      66    05H633Z    BR30ZZZ    N/A 

How can I go about this? I've tried to do a left merge but it does not give the result I am looking for.

You can combine the two dataframes on the INC_KEY column using .merge. Then, you can use .groupby() and pd.concat() to turn individual rows into the desired columns. Finally, you can drop the original "DCODE" column using .drop():

df = df1.merge(df2, on="INC_KEY", how="right")
df = df.groupby(["INC_KEY", "SEX", "AGE"]).agg({"DCODE": list}).reset_index()
df = pd.concat(
    (df, pd.DataFrame(df["DCODE"].values.tolist()).add_prefix("DCODE")), 
    axis=1
)
df = df.drop("DCODE", axis=1)

This outputs:

   INC_KEY SEX  AGE   DCODE0   DCODE1   DCODE2
0        1   F   40  BW241ZZ  BW28ZZZ  BF42ZZZ
1        2   F   24  0BH17EZ  4A103BD     None
2        3   M   66  05H633Z  BR30ZZZ     None

Here's another way:

df_out = df1.merge(df2, on='INC_KEY')
df_out = df_out.set_index(['INC_KEY', 'SEX', 'AGE', df_out.groupby('INC_KEY').cumcount()]).unstack()
df_out.columns = [f'{i}{j}' for i, j in df_out.columns]
df_out.reset_index()

Output:

   INC_KEY SEX  AGE   DCODE0   DCODE1   DCODE2
0        1   F   40  BW241ZZ  BW28ZZZ  BF42ZZZ
1        2   F   24  0BH17EZ  4A103BD      NaN
2        3   M   66  05H633Z  BR30ZZZ      NaN