<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def mean_absolute_percentage_error(df, col_true, col_pred=None):
 """ Compute mean absolute percentage error of a predicted DataFrame. Note that this method will trigger the defined flow to execute. :param df: predicted data frame :type df: DataFrame :param col_true: column name of true value :type col_true: str :param col_true: column name of predicted value, 'prediction_score' by default. :type col_pred: str :return: Mean absolute percentage error :rtype: float """

    if not col_pred:
        col_pred = get_field_name_by_role(df, FieldRole.PREDICTED_VALUE)
    return _run_evaluation_node(df, col_true, col_pred)['mape']