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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 amazon_s3_url(self, sat, band):
 """ Return an amazon s3 url the contains the scene and band provided. :param sat: Expects an object created by scene_interpreter method :type sat: dict :param filename: The filename that has to be downloaded from Amazon :type filename: String :returns: (String) The URL to a S3 file """

        if band != 'MTL':
            filename = '%s_B%s.TIF' % (sat['scene'], band)
        else:
            filename = '%s_%s.txt' % (sat['scene'], band)

        return url_builder([self.s3, sat['sat'], sat['path'], sat['row'], sat['scene'], filename])