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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _complete_url(self, url_part, registration_prefix):
 """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 """

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