Django 模板不能循环 defaultdict

import collections

data = [
  {'firstname': 'John', 'lastname': 'Smith'}, 
  {'firstname': 'Samantha', 'lastname': 'Smith'}, 
  {'firstname': 'shawn', 'lastname': 'Spencer'}, 
]

new_data = collections.defaultdict(list)

for d in data:
    new_data[d['lastname']].append(d['firstname'])

print new_data

输出如下:

defaultdict(<type 'list'>, {'Smith': ['John', 'Samantha'], 'Spencer': ['shawn']})

这是模板:

{% for lastname, firstname in data.items %}
  <h1> {{ lastname }} </h1>
  <p> {{ firstname|join:", " }} </p>
{% endfor %}

但是我模板中的循环不起作用.什么都不显示.它甚至没有给我一个错误.我怎样才能解决这个问题?它应该显示姓氏和名字，如下所示:

But the loop in my template doesn't work. Nothing shows up. It doesn't even give me an error. How can i fix this? It's supposed to show the lastname along with the firstname, something like this:

<h1> Smith </h1>
<p> John, Samantha </p>

<h1> Spencer </h1>
<p> shawn </p>