redirect output of an already running process

Long story short: you have launched your script/program but forgot to redirect the output to a file for later inspection.

#!/usr/bin/python3
#sample endless running program that prints to stdout
import time,datetime

while True:
    print(datetime.datetime.now().time())
    time.sleep(1)

Using GNU Debugger you can re-attach to the process, then invoke the creation of a logfile and duplicate the file descriptor to make the system send the data to the new file, instead of the terminal:

sudo gdb -p $(pidof python3) --batch -ex 'call creat("/tmp/stdout.log", 0600)' -ex 'call dup2(3, 1)'

note that you need to pass with the option -p the Process ID (PID) of the program; you can get it also via ps -ef

now, simply

tail -f /tmp/stdout.log

to get the output of the program.