Python loop for calculating sum of column values in pandas

I have below data frame:

a
100
200
200
b
20
30
40
c
400
50

Need help to calculate sum of values for each item and place it in 2nd column, which ideally should look like below:

a      500
100
200
200
b      90
20
30
40
c      450
400
50

Answer

If need sum by groups by column col converted to numeric use GroupBy.transform with repeated non numeric values by ffill:

s = pd.to_numeric(df['col'], errors='coerce')
mask = s.isna()
df.loc[mask, 'new'] = s.groupby(df['col'].where(mask).ffill()).transform('sum')
print (df)
    col    new
0     a  500.0
1   100    NaN
2   200    NaN
3   200    NaN
4     b   90.0
5    20    NaN
6    30    NaN
7    40    NaN
8     c  450.0
9   400    NaN
10   50    NaN

Or:

df['new'] = np.where(mask, new.astype(int), '')
print (df)
    col  new
0     a  500
1   100     
2   200     
3   200     
4     b   90
5    20     
6    30     
7    40     
8     c  450
9   400     
10   50