pyqgis - Finding variables of QgsPolygon and QgsPolyline?

now I want to use QgsPolygon (and Qgspolyline,too) class in python

I have a example by using QgsPoint but I don't know how to use in QgsPolygon

following is my Qgspoint example

    feature = QgsFeature()

    qpoint = QgsPoint( float(x), float(y) ) #get qpoint's attitube here
    feature.setGeometry(QgsGeometry.fromPoint(qpoint))#set point's geometry here
    if numpy.isnan(val):
        pass #no assign value means assign NULL value
    else:
        attr_val = round(val,4)
        feature.addAttribute(0,QVariant(attr_val))
    feature_list.append(feature)#append value here

I want to change QgsPolygon(or QgsPolyline) instead of QgsPoint but I couldn't find the variables of QgsPolygon(or QgsPolyline)

where to find the variables of QgsPolygon and QgsPolyline?

---

I have another problem here ,

  for s in dataset:
        x=s[0]
        y=s[1]      
        feature = QgsFeature()
        qpoint = QgsPoint( float(x) , float(y) )
        polygon=[]
        polygon.append(qpoint)


        feature.setGeometry(QgsGeometry.fromPolygon([polygon]))

but it's still get

        TypeError: QgsFeature.setGeometry(): argument 1 has unexpeced type 'NoneType'

how to fix it?

Have I mistake the meaning of your answer?

Answer

see Geometry Handling

1. a point: QgsPoint(x,y)


2. a line: QgsGeometry.fromPolyline([QgsPoint(x1,y1),QgsPoint(x2,y2)]))


3. a polygon: QgsGeometry.fromPolygon([[QgsPoint(x1,y1),QgsPoint(x2,y2), QgsPoint(x3,y3),QgsPoint(x1,y1)]]) -> the polygon must be closed.

thus:

line_start= QgsPoint(50,50)
line_end= QgsPoint(100,100)
line = QgsGeometry.fromPolyline([line_start, line_end])
points = [QgsPoint(60,60),QgsPoint(60,80),QgsPoint(80,80),QgsPoint(80,60),QgsPoint(60,60)] 

polygon= QgsGeometry.fromPolygon([points])

and:

feature.setGeometry(QgsGeometry.fromPolyline([line_start, line_end]))
feature.setGeometry(QgsGeometry.fromPolygon([points]))