import re

from django.core.exceptions import ImproperlyConfigured


def application_path(value):
    """A deployment-owned tenant path, never a value taken from request headers."""
    if value in ('', '/'):
        return ''
    if not re.fullmatch(r'/[a-z0-9]+(?:-[a-z0-9]+)*', value):
        raise ImproperlyConfigured('APP_BASE_PATH must be empty or /lowercase-tenant-slug')
    return value