def METHOD_NAME(self, proposal):
    """Ensure connect_url includes base_url"""
    if not proposal.value:
        # Don't add the prefix if the setting is being cleared
        return proposal.value
    urlinfo = urlparse(proposal.value)
    if not urlinfo.path.startswith(self.base_url):
        urlinfo = urlinfo._replace(path=self.base_url)
        return urlunparse(urlinfo)
    return proposal.value