So I've been attempting to use the csv module in Python to add data to existing rows and columns, but only specific columns of each row. So for examples let's say my existing csv file has the following:

id, name, city, age
1, Ed,, 34
2, Pat,, 23

So basically the city of each person is missing, so I would like to update each row with that person's city. However, the writerow method only seems replace the existing data within the csv file. Changing the open file to append mode just adds the data to a new row. Is there any way to skip the existing data, and only add the city to each row?

Thanks