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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_polyline(self, id_num, style='google'):
 """Return the polyline of the activity with the given id. :param style: The type of polyline to return. May be one of 'google', 'svg', or 'geojson'. """

        parts = ['my', 'activities', id_num, 'polyline']
        if style != 'google':
            parts.append(style)
        url = self._build_url(*parts)

        return self._json(url)