那这个呢

{% for product_in_series in series.get_products %}{%spaceless%}
    {% with product_in_series.product as product %}
    {%if not forloop.first%}
        "<img src='{% version product.get_overview 'page_image' %}'>"
        {%if not forloop.last%},{%endif%}
    {%endif%}
    {% endwith %}
{%endspaceless%}{% endfor %}

{{ forloop.last|yesno:",,"|safe }}

,-是逗号

以上都不适合我。Django3.0中的正确语法是这样的

{% with querythisandthat as A %}
   {% for  u in A %}  {{ u.interesting_stuff }} 
      {% if u == A.last %} . {% else %} ; {% endif %}
   {% endfor %}
 {% endwith %}

原因是A.last不是True/False，但它是查询集的最后一个元素
https://docs.djangoproject.com/en/3.0/ref/models/querysets/#django.db.models.query.QuerySet.first