Well, based on this[1] document. We should use the python {} statement. But, I would like to keep my python script externally in a separated file and do something like.

python filter_001 {
   /my/script/python001.py
}

log {
   ....
   filter(filter_001);
   ....
};

p.s: Btw, this sample https://www.balabit.com/documents/syslog-ng-ose-3.7-guides/en/syslog-ng-ose-guide-admin/html-single/index.html#example-python-template-functions don't work in the syslog-ng 3.9.1

[1] https://www.balabit.com/sites/default/files/documents/syslog-ng-ose-3.7-guides/en/syslog-ng-ose-v3.7-guide-admin/html-single/index.html#example-python-template-functions
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Jorge Pereira