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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 ensure_build(self):
 """ If self.config.build_url is set, ensure it's been downloaded to the builds folder. """

        if self.config.build_url:
            path = get_buildfile_path(self.config)

            # Ensure that builds_root has been created.
            mkdir(BUILDS_ROOT)
            build_md5 = getattr(self.config, 'build_md5', None)
            ensure_file(self.config.build_url, path, build_md5)

            # Now untar.
            self.untar()