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

import frappe


def execute():
	employees = frappe.get_all("Employee",
		filters={"prefered_email": ""},
		fields=["name", "prefered_contact_email", "company_email", "personal_email", "user_id"])

	for employee in employees:
		if not employee.prefered_contact_email:
			continue

		preferred_email_field = frappe.scrub(employee.prefered_contact_email)

		preferred_email = employee.get(preferred_email_field)
		frappe.db.set_value("Employee", employee.name, "prefered_email", preferred_email, update_modified=False)