Check that submitted URL is unique and allowed

clean_url.py

def clean_url(self):
    INVALID_URLS = ('about', 'admin', 'register')    
    url = self.cleaned_data['url']
    
    try:
        post = Post.objects.get(url=url)
    except Post.DoesNotExist:
        pass
    else:
        raise forms.ValidationError(u'%s already exists' % post )

    if url in INVALID_URLS:
        raise forms.ValidationError(u'%s is not a valid url' % url )

    return url