def PixelsToRaster(self, px, py, zoom):
		
		mapSize = self.tileSize << zoom
		return px, mapSize - py
Like the following with nicer default message:
dataset/ETHPy150Open Ecotrust/create-utfgrids/globalmaptiles.py/GlobalMercator.PixelsToRaster--crossover with--dataset/ETHPy150Open chromium/web-page-replay/trafficshaper_test.py/TimedTestCase.assertValuesAlmostEqual