请教 Django 中 cleaned_data 的一个用法，谢谢大家。

要上传 URL，用的自带的 URLField，但是默认是必须输入 http 开头，现在想让用户不输入。现在代码是

class PageForm(forms.ModelForm):
title = forms.CharField(max_length=128, help_text='Please enter the title')
url = forms.URLField(max_length=200, help_text='Please enter the URL')
views = forms.CharField(widget=forms.HiddenInput(), initial=0)

class Meta:
model = Page
exclude = ('category',)

#这一段不知道怎么起效果
def clean(self):
cleaned_data = self.cleaned_data
url = cleaned_data.get('url')

if url and not url.startswith('http://'):
url = 'http://' + url
cleaned_data['url'] = url

return cleaned_data

views 里的代码：
def add_page(request, category_name_slug):
try:
category = Category.objects.get(slug=category_name_slug)
except Category.DoesNotExist:
category = None

form = PageForm()
if request.method == 'POST':
form = PageForm(request.POST)
if form.is_valid():
if category:
page = form.save(commit=False)
page.category = category
page.views = 0
page.save()
return show_category(request, category_name_slug)
else:
print(form.errors)

context_dict = {'form': form, 'category': category}
return render(request, 'rango/add_page.html', context_dict)