it is not realy complicated, look at Memory provider in vector: :

• a point is created with QgsPoint(x,y) and QgsGeometry.fromPoint(QgsPoint(x,y))
• a line is created with two points: QgsGeometry.fromPolyline([QgsPoint(x1,y1),QgsPoint(x2,y2)]))
• a polygon is created with a list of points: QgsGeometry.fromPolygon([[QgsPoint(x1,y1),QgsPoint(x2,y2), QgsPoint(x3,y3)]])

1. two points:

   create a memory layer with two points

   layer = QgsVectorLayer('Point', 'points' , "memory") pr = layer.dataProvider()

   add the first point

   pt = QgsFeature() point1 = QgsPoint(50,50) pt.setGeometry(QgsGeometry.fromPoint(point1)) pr.addFeatures([pt])

   update extent of the layer

   layer.updateExtents()

   add the second point

   pt = QgsFeature() point2 = QgsPoint(100,150) pt.setGeometry(QgsGeometry.fromPoint(point2)) pr.addFeatures([pt])

   update extent

   layer.updateExtents()

   add the layer to the canvas

   QgsMapLayerRegistry.instance().addMapLayers([layer])

2. the line connecting the two points

   layer = QgsVectorLayer('LineString', 'line' , "memory") pr = layer.dataProvider() line = QgsFeature() line.setGeometry(QgsGeometry.fromPolyline([point1,point2])) pr.addFeatures([line]) layer.updateExtents() QgsMapLayerRegistry.instance().addMapLayers([layer])

3. a polygon covering the points

   layer = QgsVectorLayer('Polygon', 'poly' , "memory") pr = layer.dataProvider() poly = QgsFeature() points = [point1,QgsPoint(50,150),point2,QgsPoint(100,50)]

   or points = [QgsPoint(50,50),QgsPoint(50,150),QgsPoint(100,150),QgsPoint(100,50)]

   poly.setGeometry(QgsGeometry.fromPolygon([points])) pr.addFeatures([poly]) layer.updateExtents() QgsMapLayerRegistry.instance().addMapLayers([layer])

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Changes in QGIS 3.0 and onward:

For QGIS 3.0 and onward the above workflow is still correct, but certain functions have changed. See https://qgis.org/api/api_break.html

To update the above code, change following functions:

QgsPoint -> QgsPointXY
QgsfromPoint -> QgsfromPointXY
QgsfromPolyline -> QgsfromPolylineXY
QgsfromPolygon -> QgsfromPolylineXY
QgsfromPolyline -> QgsfromPolylineXY
QgsMapLayerRegistry -> QgsProject