Python3: Continuously update datetime value from inside while loop - function

Question

I'm having trouble with the code below which is asking the user to enter a 'wakeup' time, which the python script will then use to calculate how long until the clock reaches zero. From what I have below, I think that I need to place the tdelta calculation inside the while loop so that it constantly checks for the current time.
At the moment, It seems that tdelta will check for the current time in seconds and then use that same value to run through the while loop. Therefore never ending because it uses the same time over and over again. Should I be using a function inside of the while loop to continuously check for the new value and then evaluate true or false?
from datetime import datetime
import time
now = datetime.now()
hms = "%s:%s:%s" % (now.hour, now.minute, now.second)
ans = input('Enter hour:minute:seconds')
s1 = hms
s2 = ans # for example
FMT = '%H:%M:%S'
tdelta = datetime.strptime(s2, FMT) - datetime.strptime(s1, FMT)
if tdelta.days < 0:
tdelta = timedelta(days=0,
seconds=tdelta.seconds, microseconds=tdelta.microseconds)
while tdelta.seconds != 0:
# Use this to see what is happening inside the loop. Here it continuously prints the same time in seconds so is a continuous loop. I need to somehow update the tdelta time.
if tdelta.seconds != 0:
print(tdelta.seconds)
time.sleep(1)
else:
print('time up...do something')
I've tried dozens of variations of the code above but with no luck. I appreciate any tips. Thanks.

Answer 1

Thanks to Padraic Cunningham who provided the missing code to get this working.
Step one: Import timedelta from datetime
Step two: add tdelta -= timedelta(seconds=1) to the first line under the while loop.
from datetime import datetime
from datetime import timedelta
import time
now = datetime.now()
hms = "%s:%s:%s" % (now.hour, now.minute, now.second)
ans = input('Enter hour:minute:seconds')
s1 = hms
s2 = ans # for example
FMT = '%H:%M:%S'
tdelta = datetime.strptime(s2, FMT) - datetime.strptime(s1, FMT)
if tdelta.days < 0:
tdelta = timedelta(days=0,
seconds=tdelta.seconds, microseconds=tdelta.microseconds)
while tdelta.seconds != 0:
tdelta -= timedelta(seconds=1)
"""Use this to see what is happening inside the loop. Here it continuously prints the same time in seconds so is a continuous loop. I need to somehow update the tdelta time."""
if tdelta.seconds != 0:
print(tdelta.seconds)
time.sleep(1)
else:
print('time up...do something')
The end result is that the command line will count down to zero and once tdelta IS equal to 0, it will print from the else statement.