'This method is used to defer the construction of the final url in
the case that the Api is created with a Blueprint.
:param url_part: The part of the url the endpoint is registered with
:param registration_prefix: The part of the url contributed by the
blueprint.  Generally speaking, BlueprintSetupState.url_prefix'

def _complete_url(self, url_part, registration_prefix):

    parts = {'b': registration_prefix, 'a': self.prefix, 'e': url_part}
    return ''.join((parts[key] for key in self.url_part_order if parts[key]))
