def METHOD_NAME(key, data, errors, context):
    # Get the current extras index
    current_indexes = [k[1] for k in data.keys()
                       if len(k) > 1 and k[0] == 'extras']
    new_index = max(current_indexes) + 1 if current_indexes else 0
    data[('extras', new_index, 'key')] = key[-1]
    data[('extras', new_index, 'value')] = data[key]