simple list check with loops

[Low_Ki_]

Ok so now my question is, how would I retrieve dict data from a file, update it to a dict in my program, then write the new username / password recieved to the end (append) of the dict in the file as a key:value pair?

e.g with a list I would retrieve data like so:

usrdatalist = []

# Open file to read
with open("usrdata", "r") as usrinfo:
    for l in usrinfo:
        un, pw = l.split(',')
        usrdatalist.append([un, pw])

and then write the new data after getting the username and password info from user  like so:

with open("usrdata", "a") as usrinfo:
    usrinfo.write(username+','+password+'\n')

How would I go about changing this to fit a dict?