Then try it with a file in the same directory as your script:
f = open("test.txt")
Or try:
f = open("./test.txt")
f = open("c:/test.txt")
f = open("c:/temp/test.txt")
f = open("test.txt")
Or try:
f = open("./test.txt")
f = open("c:/test.txt")
f = open("c:/temp/test.txt")