calculate percentage based on specific column value

Question

I want to calculate percentage for each row. Below is an example dataframe:

    KEY  DESCR  counts
0   2    to A   1
1   2    to B   1
2   20   to C   1
3   35   to D   2
4   110  to E   4
5   110  to F   1
6   110  to G   1

percentage formula is: (counts / sum of counts.indicator on column KEY)*100
Example: (1/2)*100

below is a stuck code since i try many times but not happen.

percentage = []

for i in range(len(df)):
    percentage.append((df['counts'][i] / ...............) * 100) 

df['PERCENTAGE'] = percentage 
df

Expected output is:

    KEY  DESCR  counts  PERCENTAGE
0   2    to A   1       50
1   2    to B   1       50
2   20   to C   1       100
3   35   to A   2       100
4   110  to E   4       67
5   110  to C   1       16
6   110  to G   1       16

Can anyone help me to solve this. Thank you

Answer 1

If performance is important use GroupBy.transform with sum and division original column by Series.div , last multiple by Series.mul :

df['PERCENTAGE'] = df['counts'].div(df.groupby('KEY')['counts'].transform('sum')).mul(100)

You can divide each value per groups, but if large DataFrame or many groups is is less effective:

df['PERCENTAGE'] = df.groupby('KEY')['counts'].transform(lambda x: x / x.sum()).mul(100)

print (df)
   KEY DESCR  counts  PERCENTAGE
0    2  to A       1   50.000000
1    2  to B       1   50.000000
2   20  to C       1  100.000000
3   35  to D       2  100.000000
4  110  to E       4   66.666667
5  110  to F       1   16.666667
6  110  to G       1   16.666667