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I'm working with polygon coordinates in QGIS using PyQGIS using ESRI shapefiles as input data. My question is, are the polygon vertices (I'm referring only to the outer boundary in the case of polygons with holes) stored as clockwise or as counterclockwise in a shapefile or can be in mixed order? Is the order always the same or can be different depending on the source of the data: digitizing, converting from other sources as *.csv files, etc.?