This is sadly Linux Only, start by installing inotify-tools:
# apt-get install inotify-tools
and create the following script:
#!/bin/sh clear echo Waiting for filechange ... inotifywait -q -m -e close_write,moved_to --format %e/%f . | while IFS=/ read -r events file; do if [ "$file" = "script.py" ]; then clear python ./script.py fi done
this actually watches the directory, because Vim writes files by writing your alterations to a temporary file and moving it in place of the old one, however this aproach also works with normal write.