geometry - How to draw polygons from the python console?

I've been visiting and revisiting the page on geometry handling in the PyQGIS Cookbook: http://documentation.qgis.org/2.0/en/docs/pyqgis_developer_cookbook/geometry.html but can't seem to figure out how to get the polygon to draw from the Python console. Can anyone help?

Answer

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