What Does A Python Process Return Code -9 Mean?

I have a python script which returns the exit status of -9. I tried to get the root of the problem with the atexit module, but it does not get called. Any hints to help me find why

Solution 1:

The script was killed by the operating system. Negative return values are the signal number which was used to kill the process.

The script needed too much memory. I found this in syslog:

Out of memory: Kill process 26184 (python) score 439 or sacrifice child
Killed process 26184 (python) total-vm:628772kB, anon-rss:447660kB, file-rss:0kB