How to add rows in one column based on repeated values in another column , and finally keep the first row in python?

Question

I am very new to the python pandas module.

Suppose I have a data frame or table as follows:

 df = pd.DataFrame({
        'Column A': [12,12,12, 15, 16, 141, 141, 141, 141],
         'Column B':['Apple' ,'Apple' ,'Apple' , 'Red', 'Blue', 'Yellow', 'Yellow', 'Yellow', 'Yellow'],
        'Column C':[100, 50, np.nan , 23 , np.nan , 199 , np.nan , 1,np.nan]
    }) 

or I have a data table as follows:

    | Column A | Column B |Column C 
----| -------- | ---------|--------
0   | 12       | Apple    |100     
1   | 12       | Apple    |50      
2   | 12       | Apple    |NaN      
3   | 15       | Red      |23       
4   | 16       | Blue     |NaN      
5   | 141      | Yellow   |199      
6   | 141      | Yellow   |NaN      
7   | 141      | Yellow   |1        
8   | 141      | Yellow   |NaN  

• If values in column A are repeated then add the corresponding values in Column C and paste the sum in new column D (For example, there are 3 rows for 12, thus we should add corresponding values 100 + 50 + NaN, and the sum result 150 should be stored in new column D).

• If values in column A do not repeat directly paste Column C values in new column D (such as row 3) but for NaN, it should be 0 (such as row 4).

Could you please help me to get an output like this in python jupyter notebook:

      | Column A | Column B |Column C |Column D 
----- | -------- | ---------|---------|---------
 0    | 12       | Apple    |100      |150      
 1    | 15       | Red      |23       |23       
 2    | 16       | Blue     |NaN      |0        
 3    | 141      | Yellow   |199      |200  

Answer 1

df.groupby("Column A", as_index=False).agg(B=("Column B", "first"),
                                           C=("Column C", "first"),
                                           D=("Column C", "sum"))
#      Column A         B         C         D
# 0          12     Apple     100.0     150.0
# 1          15       Red      23.0      23.0
# 2          16      Blue       NaN       0.0
# 3         141    Yellow     199.0     200.0