flask中flash不显示

需要在html文件中body中加入下列语句

<div class='container'>
    <div class="row">
        {% with messages = get_flashed_messages() %}
            {% for message in messages %}
                {% if message %}
                    <div class="alert alert-warning" role="alert">{{ message }}</div>
                {% endif %}
            {% endfor %}
        {% endwith %}
        {% if form and form.errors %}
            {% for key, error in form.errors.items() %}
                <div class="alert alert-warning" role="alert">{{ error }}</div>
            {% endfor %}
        {% endif %}
    </div>
</div>