What's wrong with this little python script?

Question

I followed this tutorial to log my CPU temperature into a .csv-file and try to run the .py script it at bootup with cron.

The script:

from gpiozero import CPUTemperature
from time import sleep, strftime, time
cpu = CPUTemperature()
with open("/home/user/Desktop/cpu_log.csv", "a") as log:
    while True:
        temp = cpu.temperature
        log.write("{0},{1}\n".format(strftime("%Y-%m-%d %H:%M:%S"),str(temp)))
        sleep(300)

I did a chmod 757 to the python script. Then added it to crontab -e: @reboot python3 /home/user/Desktop/log.py

If I execute python3 log.py from shell when the machine is running, the .csv file is being created and written to perfectly. However when I reboot my machine, I see the script running with ps -aux | grep log.py. And I see the .csv file being created once. However not a single line is written to the file and after I delete it, it is not recreated. There is just an empty csv file on the desktop.

Answer 1

You never flush the buffer, thus, the info is not being dumped to the file. This is done when the file is closed (you never do it), assigning a buffer size (see this stackoverflow answer and comments or flushing the buffer. To achieve the latter (and the easier IMHO), add before the sleep line:

log.flush()

It should look like:

    temp = cpu.temperature

    log.write("{0},{1}\n".format(strftime("%Y-%m-%d %H:%M:%S"),str(temp)))
    log.flush()

    sleep(300)

Answer 2

You failed to tell the cron user where python3 is located. cron's path is different than yours.

@reboot /usr/bin/python3 /home/user/Desktop/log.py

EDIT: As I have learned here recently, there are some "vagaries" associated with cron's concept of the @reboot event. To quote my mentor:

"read man crontab. It says: "Please note that startup, as far as @reboot is concerned, is the time when the cron(8) daemon startup. In particular, it may be before some system daemons, or other facilities, were startup. This is due to the boot order sequence of the machine." So you may run into problems if your program needs specific services."

If your script doesn't seem to run properly under @reboot, and that is due to the cause cited above, you can try to remedy that by delaying when cron starts your script. The other thing you should consider is redirecting the stderr output from cron to a file. All that said, your crontab entry will now look like this (make sure you get the entire line):

@reboot (sleep 30; /usr/bin/python3 /home/user/Desktop/log.py > /home/pi/cronjoblog 2>&1)

If you still have issues, check the contents of /home/pi/cronjoblog for clues.