Change date formats in csv with Python 3

import csv import re from datetime import datetime lines = [] # open file as read-only with open('partner_new_testteam.csv', "r", newline='') as data: reader = csv.reader(data) # go over all of its rows, and the row's items and change # items that match the date format for row in reader: for i, string in enumerate(row): if re.match(r"\d+\/\d+\/\d+", string): datetimeobject = datetime.strptime(string, '%d/%m/%Y') new_string = datetimeobject.strftime('%Y-%m-%d') row[i] = new_string print("Replaced", string, "with", new_string) # save edited, and originally correct ones to new list new_row = row lines.append(new_row) # write new rows by overwriting original file with open('partner_new_testteam.csv', "w", newline='') as data: writer = csv.writer(data) writer.writerows(lines) 

Your current code does not actually change anything. You never replaced anything and you didn't open the file with write access.

You also should not use try: like it is an if. The regex matches x/x/x, where x is any amount of numbers.