Simple Python script to convert name value to tabular format

#!/usr/bin/env python
import csv

file1reader = csv.reader(open("mapping.csv"), delimiter=",")

header1 = file1reader.next() #header
header2 = file1reader.next() #header

data = dict()
result=[]
row=[]
for Key, Prop, Val, Flag in file1reader:
        if key in data: # found it
        x=data[Key]
        data[Key] = x + "," + Val
        row.append(Val)
    else:
        data[Key] = Val
        result.append(row)
        row=[]
        row.append(Key)
        row.append(Val)
   
with open('myfile.csv', 'wb') as f:
    w = csv.writer(f,dialect='excel')
    w.writerows(result)   

Example mapping.csv file:

Key1, prop1, value1
Key1, prop2, value2
Key1, prop3, value3
Key2, prop1, value4
Key2, prop2, value5
Key2, prop3, value6

Output file:
Key1, value1, value2, value3
Key2, value3, value4, value5