Hello E.M.

To write a on_change method in .py file like this,

def on_change_emp_dept(self, cr, uid, ids, employee_id, context=None):
dept_id = False
if employee_id:
emp_pool = self.pool.get("hr.employee")
emp_obj = emp_pool.browse(cr, uid, employee_id, context=context)
dept_id = emp_obj and emp_obj[0].department_id.id or False
return {'value':{'department_id':dept_id}}

<field name="employee_id" on_change="on_change_emp_dept(employee_id)" />