Pandas : If a column is having duplicates then copy values of corresponding column and copy it to new column

Question

If my dataframe is like this,

 X Y Z

 1 a

 1 b

 2 c

the output should be

X Y Z

1 a a,b

1 b a,b

2 c

Condition:

If a X has duplicates then it should all take the values of Y of that X duplicate and convert to csv values and paste in column Z

user_18694400 · Answer

df["Z"] = df.X.map(df.groupby("X").agg(list).apply(lambda x: "" if len(x.Y) == 1 else ",".join(x.Y), axis=1))

user_16343464 · Answer

Use a groupby.transform and mask:

g = df.groupby('X')['Y']
df['Z'] = g.transform(','.join).mask(g.transform('size')==1, '')

output:

   X  Y    Z
0  1  a  a,b
1  1  b  a,b
2  2  c