Writing a python program with output in a spreadsheet

I need to write a python program which outputs to a spreadsheet. In pseudo-code, the program will look roughly like this:

folder = 'User/Path/FolderName'
for file in folder:
    return filename
    functionA(file) #returns OutputA
    functionB(file) #returns OutputB
    functionC(file) #returns OutputC

These functions are already written in Python, so I'd prefer to stay in Python. Ideally, the output would be a spreadsheet (.csv, Excel, tab-delimited txt, whatever) that will look something like this:

FileName   A          B          C 
file1      OutputA1   OutputB1   OutputC1
file2      OutputA2   OutputB2   OutputC2

Also ideally, I would be able to run the program again on a different folder of files of data, and add those results to the spreadsheet later.

I found these other questions here and here, as well as a few packages such as Pyspread, but I'm not really sure how to go about my problem.

So after looking at the csv package, I think I would do something like this:

import csv
with open('data.csv', 'wb') as data:
    datafile = csv.writer(data, delimiter=',',
                        quotechar='|', quoting=csv.QUOTE_MINIMAL)
for file in folder:
    datafile.writerow(file, functionA(file), functionB(file), functionC(file))

Just to be clear, the "wb" means I can write to the file, correct? Also, are the quotechar= and quoting= required syntax?

import csv
numbers = [1,2,3,4]

def functionA(x):
    return x + 2
def functionB(x):
    return x * 3
def functionC(x):
    return x - 7

with open('data.csv', 'wb') as data:
    datafile = csv.writer(data, delimiter=',',
                      quotechar='|', quoting=csv.QUOTE_MINIMAL)
    for n in numbers:
        a = functionA(n)
        b = functionB(n)
        c = functionC(n)
        datafile.writerow([n, a, b, c])

This does what I expected it to, namely put this into the csv file:

1   3   3   -6
2   4   6   -5
3   5   9   -4
4   6   12  -3

However, if I run it again with a new set of numbers, it overwrites the old data. I'd like to keep adding new rows to the spreadsheet. How would I do this?