def clean_url(self):
        url = self.cleaned_data['url']
        if not url.startswith('/'):
            raise forms.ValidationError(
                ugettext("URL is missing a leading slash."),
                code='missing_leading_slash',
            )
        if (settings.APPEND_SLASH and
                'django.middleware.common.CommonMiddleware' in settings.MIDDLEWARE_CLASSES and
                not url.endswith('/')):
            raise forms.ValidationError(
                ugettext("URL is missing a trailing slash."),
                code='missing_trailing_slash',
            )
        return url
dataset/ETHPy150Open django/django/django/contrib/flatpages/forms.py/FlatpageForm.clean_url/original