Return to Answer

Your code is working for me without problem. I've try it in the QGIS python console.

When i run it with a first start and end point, i get the red line in the cable layer then i run the script with others start and end point and i get the yellow line added in the same layer.  

and what i get in the attribute table (length and geom calculate with the field calculator)

I just remove the part of the code using QgsRubberBand as it was useless. The code I used:

from PyQt4.QtCore import *
from PyQt4.QtGui import *
from qgis.core import *
from qgis.gui import *
from qgis.networkanalysis import *

#building the graph**
vl = qgis.utils.iface.mapCanvas().currentLayer()
director = QgsLineVectorLayerDirector(vl, -1, '', '', '', 3)
properter = QgsDistanceArcProperter()
director.addProperter(properter)
crs = qgis.utils.iface.mapCanvas().mapRenderer().destinationCrs()
builder = QgsGraphBuilder(crs)

#coordinates of the Start point and endPoint** 
pStart = QgsPoint(-1.041,0.916)
pStop = QgsPoint(-0.813,0.152)
tiedPoints = director.makeGraph(builder, [pStart, pStop])
graph = builder.graph()

#Calculation of the shortest path**    
tStart = tiedPoints[0]
tStop = tiedPoints[1]

idStart = graph.findVertex(tStart)
idStop = graph.findVertex(tStop)

(tree, cost) = QgsGraphAnalyzer.dijkstra(graph, idStart, 0)

if tree[idStop] == -1:
  print "Path not found"
else:
  p = []
  curPos = idStop
  while curPos != idStart:
    pnt = graph.vertex(graph.arc(tree[curPos]).inVertex()).point()

    p.append(pnt)

    curPos = graph.arc(tree[curPos]).outVertex()

  p.append(tStart)

 #I build here a polyline based on the points in the p[] array
geom = QgsGeometry.fromPolyline(p)


 # I add here my result in a memory layer called cable and i want to do it for every feature i get after transforming the qgsrubberband object     

v_layer = None
for layer in QgsMapLayerRegistry.instance().mapLayers().values():
   if layer.name() == 'cable':
        v_layer = layer
if v_layer is None: 
    v_layer = QgsVectorLayer("LineString", "cable", "memory")
    v_layer.addAttribute(QgsField("id", QVariant.String))
    v_layer.addAttribute(QgsField("type", QVariant.String))
    QgsMapLayerRegistry.instance().addMapLayers([v_layer])

pr = v_layer.dataProvider()
v_layer.startEditing()


seg = QgsFeature()
seg.setGeometry(geom)
pr.addFeatures( [seg] )


v_layer.commitChanges()

Your code is working for me without a problem. I've tried it in the QGIS Python Console.

When I run it with a first start and end point, I get the red line in the cable layer then I run the script with other start and end points and I get the yellow line added in the same layer. 

and what I get in the attribute table (length and geom calculated with the field calculator)

I just removed the part of the code using QgsRubberBand as it was useless.

The code I used:

from PyQt4.QtCore import *
from PyQt4.QtGui import *
from qgis.core import *
from qgis.gui import *
from qgis.networkanalysis import *

#building the graph**
vl = qgis.utils.iface.mapCanvas().currentLayer()
director = QgsLineVectorLayerDirector(vl, -1, '', '', '', 3)
properter = QgsDistanceArcProperter()
director.addProperter(properter)
crs = qgis.utils.iface.mapCanvas().mapRenderer().destinationCrs()
builder = QgsGraphBuilder(crs)

#coordinates of the Start point and endPoint** 
pStart = QgsPoint(-1.041,0.916)
pStop = QgsPoint(-0.813,0.152)
tiedPoints = director.makeGraph(builder, [pStart, pStop])
graph = builder.graph()

#Calculation of the shortest path**    
tStart = tiedPoints[0]
tStop = tiedPoints[1]

idStart = graph.findVertex(tStart)
idStop = graph.findVertex(tStop)

(tree, cost) = QgsGraphAnalyzer.dijkstra(graph, idStart, 0)

if tree[idStop] == -1:
  print "Path not found"
else:
  p = []
  curPos = idStop
  while curPos != idStart:
    pnt = graph.vertex(graph.arc(tree[curPos]).inVertex()).point()

    p.append(pnt)

    curPos = graph.arc(tree[curPos]).outVertex()

  p.append(tStart)

 #I build here a polyline based on the points in the p[] array
geom = QgsGeometry.fromPolyline(p)


 # I add here my result in a memory layer called cable and i want to do it for every feature i get after transforming the qgsrubberband object     

v_layer = None
for layer in QgsMapLayerRegistry.instance().mapLayers().values():
   if layer.name() == 'cable':
        v_layer = layer
if v_layer is None: 
    v_layer = QgsVectorLayer("LineString", "cable", "memory")
    v_layer.addAttribute(QgsField("id", QVariant.String))
    v_layer.addAttribute(QgsField("type", QVariant.String))
    QgsMapLayerRegistry.instance().addMapLayers([v_layer])

pr = v_layer.dataProvider()
v_layer.startEditing()


seg = QgsFeature()
seg.setGeometry(geom)
pr.addFeatures( [seg] )


v_layer.commitChanges()

Your code is working for me without problem. I've try it in the QGIS python console.

Your code is working for me without problem. I've try it in the QGIS python console.

Your code is working for me without problem. I've try it in the QGIS python console.

Your code is working for me without problem. I've try it in the QGIS python console.