django消息没有显示

'django.contrib.sessions.middleware.SessionMiddleware',
'django.contrib.messages.middleware.MessageMiddleware',

INSTALLED_APPS = [
    # ...
    'django.contrib.messages',
    # ...
]

{% if messages %}
<ul class="messages">
    {% for message in messages %}
    <li{% if message.tags %} class="{{ message.tags }}"{% endif %}>{{ message }}</li>
    {% endfor %}
</ul>
{% endif %}

from django.contrib import messages

def my_view(request):
    # ...
    messages.success(request, 'This is a message.')
    # ...