Django/venv/lib/python3.7/site-packages/django/contrib/flatpages/forms.py

from django import forms
from django.conf import settings
from django.contrib.flatpages.models import FlatPage
from django.utils.translation import gettext, gettext_lazy as _


class FlatpageForm(forms.ModelForm):
    url = forms.RegexField(
        label=_("URL"),
        max_length=100,
        regex=r'^[-\w/\.~]+$',
        help_text=_("Example: '/about/contact/'. Make sure to have leading and trailing slashes."),
        error_messages={
            "invalid": _(
                "This value must contain only letters, numbers, dots, "
                "underscores, dashes, slashes or tildes."
            ),
        },
    )

    class Meta:
        model = FlatPage
        fields = '__all__'

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        if not self._trailing_slash_required():
            self.fields['url'].help_text = _(
                "Example: '/about/contact'. Make sure to have a leading slash."
            )

    def _trailing_slash_required(self):
        return (
            settings.APPEND_SLASH and
            'django.middleware.common.CommonMiddleware' in settings.MIDDLEWARE
        )

    def clean_url(self):
        url = self.cleaned_data['url']
        if not url.startswith('/'):
            raise forms.ValidationError(
                gettext("URL is missing a leading slash."),
                code='missing_leading_slash',
            )
        if self._trailing_slash_required() and not url.endswith('/'):
            raise forms.ValidationError(
                gettext("URL is missing a trailing slash."),
                code='missing_trailing_slash',
            )
        return url

    def clean(self):
        url = self.cleaned_data.get('url')
        sites = self.cleaned_data.get('sites')

        same_url = FlatPage.objects.filter(url=url)
        if self.instance.pk:
            same_url = same_url.exclude(pk=self.instance.pk)

        if sites and same_url.filter(sites__in=sites).exists():
            for site in sites:
                if same_url.filter(sites=site).exists():
                    raise forms.ValidationError(
                        _('Flatpage with url %(url)s already exists for site %(site)s'),
                        code='duplicate_url',
                        params={'url': url, 'site': site},
                    )

        return super().clean()