def get_tile_url(self, zoom, x, y):
        """
        Fill the placeholders of the tile url with zoom, x and y.

        >>> ms = MapSource.from_xml("mapsources/osm.xml")
        >>> ms.get_tile_url(42, 43, 44)
        'http://tile.openstreetmap.org/42/43/44.png'
        """
        return self.tile_url.format(**{"$z": zoom, "$x": x, "$y": y})