Example for an on-add hook that sets the project of newly added tasks to the current directory's name by default.

on-add.project_pwd.py

#!/usr/bin/env python
import json
import os
import sys

t = json.loads(sys.stdin.readline())

if not 'project' in t:
    t['project'] = os.path.basename(os.getcwd())

print(json.dumps(t))