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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 get_rendition_url(self, width=0, height=0):
        '''get the rendition URL for a specified size

        if the renditions does not exists it will be created
        '''
        if width == 0 and height == 0:
            return self.get_master_url()

        target_width, target_height = self.get_rendition_size(width, height)

        key = '%sx%s' % (target_width, target_height)
        if not self.renditions:
            self.renditions = {}
        rendition_name = self.renditions.get(key, False)
        if not rendition_name:
            rendition_name = self.make_rendition(target_width, target_height)
        return default_storage.url(rendition_name)