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

import frappe

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

    # add "Completed" if customized
    for doctype in ('Task', 'Project Task'):
        property_setter_name = frappe.db.exists('Property Setter', dict(doc_type = doctype, 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"')