get the middle points of lines in Qgis

qgis_midpoint.py

from qgis.core import *
import qgis.utils


def midpair(plist):
    '''Iterate over pairs in a list '''
    x=[]
    y=[]
    for c in plist:
        x.append(c[0])
        y.append(c[1])
    if len(plist) ==2:
        mx = (max(x)+min(x))/2
        my = (max(y)+min(y))/2
    elif len(plist) % 2 == 1:
        mx = x[(len(plist)+1)/2]
        my = y[(len(plist)+1)/2]
    else:
        mx = (x[len(plist)/2] + x[(len(plist)/2)+1])/2
        my = (y[len(plist)/2] + y[(len(plist)/2)+1])/2
    return QgsPoint(mx, my)


def create_geometry(point,pr):
    # create geometry record
    seg = QgsFeature()    
    #seg.setAttribute([name])
    seg.setGeometry(QgsGeometry.fromPoint(point))
    pr.addFeatures( [seg] )
        
s12p = QgsVectorLayer("Point", "s12-16midpoint_2", "memory")
pr = s12p.dataProvider()
pr.addAttributes([QgsField('combo')])


s12 = qgis.utils.iface.activeLayer()
iter = s12.getFeatures()
c = 0
for f in iter:
    g= f.geometry()
    m_p = midpair(g.asPolyline())
    #g.asPolyline() return a list of points in the line; 
    #could be more than 2 points(when the line is curvy)
    create_geometry(m_p,pr)
    s12p.updateExtents()
QgsMapLayerRegistry.instance().addMapLayers([s12p])
print ('done')