在Django表单上引发ValidationError的问题

我正在尝试验证数据库中是否已存在提交的URL.

Form类的相关部分如下所示:

from django.contrib.sites.models import Site
class SignUpForm(forms.Form):
    # ... Other fields ...
    url = forms.URLField(label='URL for new site, eg: example.com')

    def clean_url(self):
        url = self.cleaned_data['url']
        try:
            a = Site.objects.get(domain=url)

        except Site.DoesNotExist:
            return url

        else:
            raise forms.ValidationError("That URL is already in the database.  Please submit a unique URL.")

    def clean(self):
        # Other form cleaning stuff.  I don't *think* this is causing the grief

问题是,无论我提交什么价值,我都无法提出ValidationError.如果我在clean_url()方法中做了类似的事情:

if Site.objects.get(domain=url):
    raise forms.ValidationError("That URL is already in the database.  Please submit a unique URL.")

然后我收到一个DoesNotExist错误,即使对于已经存在于数据库中的URL也是如此.有任何想法吗？