Simple, using inotifywait (install your distribution's inotify-tools package):
while inotifywait -e close_write myfile.py; do ./myfile.py; done
This has a big limitation: if some program replaces myfile.py with a different file, rather than writing to the existing myfile, inotifywait will die. Most editors work that way.
To overcome this limitation, use inotifywait on the directory:
while true; do
change=$(inotifywait -e close_write,moved_to,create .)
change=${change#./ * }
if [ "$change" = "myfile.py" ]; then ./myfile.py; fi
done