Question : write to file

hi i would like to know how do you write to a file but not overwriting it completely

eg:
testfile
james:1111

i wish to write it so it becomes james:4444 but it keeps overwriting the whole file

for line in open("testfile", "w").readlines():
     list = line.strip().split(':')

newinput = raw_input("Enter number")
list[1] = newinput

if (newinput == list[1])
   print "success"

any help please ? thanks

Answer : write to file

If you use an open mode of "r+", you can update the file.  Even with that, I don't think the code you showed will do what you specified, though.