brotherton-erpnext/erpnext/patches/v12_0/set_task_status.py

import frappe

def execute():
	frappe.reload_doctype('Task')

	# add "Completed" if customized
	property_setter_name = frappe.db.exists('Property Setter', dict(doc_type='Task', field_name = 'status', property = 'options'))
	if property_setter_name:
		property_setter = frappe.get_doc('Property Setter', property_setter_name)
		if not "Completed" in property_setter.value:
			property_setter.value = property_setter.value + '\nCompleted'
			property_setter.save()

	# renamed default status to Completed as status "Closed" is ambiguous
	frappe.db.sql('update tabTask set status = "Completed" where status = "Closed"')