Question

while True:
    now = datetime.datetime.now()
    if now.second == 1:
        print "One"

my program print One about 7 times. How do I make it print only once?

Answer 1

Your computer is too fast.

It takes the current time, tests whether the current second is one and then again. And since it is so fast it can do this within less than one second, you get more lines of output.

Make it wait after each iteration:

while True:
    now = datetime.datetime.now()
    if now.second == 1:
        print "One"
        time.sleep(59) # wait 59 seconds after success
    time.sleep(1) # wait 1 second after each fail

This program will sleep most time. If you want it to do anything useful, it will be a different program.