First time with Pythonista here and I cannot for the life of me figure out how I can point my script to a csv file I have saved on my iPhone

I tried both Pythonista/External Files / test.csv
and also Pythonista/Script Library/ This iPhone / test.csv

An excerpt of my code below:

row = ['a', 'b', 'c']

with open(''*WHAT IS EASIEST FILE PATH TO PUT HERE WRITE TO A LOCAL CSV.csv 'a') as csvFile:
writer = csv.writer(csvFile)
for i in b:
row = ['a', i, 'c']
writer.writerow(row)

My goal is to use Siri shortcut to run Pythonista in which it will modify a csv file on my iPhone.

Thanks!