QGIS 2.18: Speed up QgsFeatureRequest in Python Script

Question

I have two input layers. The first one is a line-layer named "graph" and the second one is a point-layer named "nodes".

The "graph"-layer:

The graph layer holds 177350 line features which are representing a streetmap. Loaded into QGIS it looks like this:

Each street (or line) holds some information: The id of the street (edgeid), the node id of the startnode, the node id of the endnode and some other attributes as shown below:

The "nodes"-layer:

The nodes layer holds 87667 point features which are representing the start- or endnodes of the streets in the graph-layer. Loaded into QGIS and overlayed with the graph-layer it looks like this:

Every node in the point-layer has a unique nodeid, x-, y- coordinates and a type (the type attribute is not relevant):

What I want to do is to calculate the azimuth for every street (line) in the graph layer. The outputlayer of my python script should look exactly like the graph-layer, just with an additional azimuth attribute. Like this:

To achieve this, I have written an python script which loops through all the features of the graph layer. For every street, I look up the x-,y-coordinates of the startnode, and of the endnode. I will need the x-,y-coordinates for calculate the azimuth later on in the script. I am doing this using QgsFeatureRequest. But as there are 177350 lines to go through, I will have to request 177350 * 2 times (177350 times for startnodes and 177350 times for endnodes). And it just takes too long.

I have already tried to set the "NoGeometry"-Flag and the setSubsetOfAttributes-method as it was suggested here. But it seems to be useless. What am I doing wrong here? How can I speed up the process of searching for the nodeids in the nodes-layer?

i = 0
n = len(feats)

# loop through graph layer and find node coordinates for from/to nodes
for i, feat1 in enumerate(feats):
    progress.setPercentage(int(100 * i / n))
    fromNodeId = feat1.attributes()[idx_from]
    toNodeId = feat1.attributes()[idx_to]

    # search for fromNode by fromNodeId
    exp = QgsExpression('nodeid = \'' + fromNodeId +'\' ')
    request = QgsFeatureRequest(exp)
    request.setFlags(QgsFeatureRequest.NoGeometry)
    request.setSubsetOfAttributes(['nodeid', 'x', 'y'], inputNodes.fields())
    request.setLimit(1)
    print "time 1"

    for feat2 in inputNodes.getFeatures(request):
        from_x = feat2.attributes()[idx_x]
        from_y = feat2.attributes()[idx_y]
        print "time 2"

    # search for specific ToNode by ToNodeId
    exp = QgsExpression('nodeid = \'' + toNodeId +'\' ')
    request = QgsFeatureRequest(exp)
    request.setFlags(QgsFeatureRequest.NoGeometry)
    request.setSubsetOfAttributes(['nodeid', 'x', 'y'], inputNodes.fields())
    request.setLimit(1)
    print "time 3"

    for feat2 in inputNodes.getFeatures(request):
        to_x = feat2.attributes()[idx_x]
        to_y = feat2.attributes()[idx_y]
        print "time 4"

    fromPoint = QgsPoint(from_x, from_y)
    toPoint = QgsPoint(to_x, to_y)


    newLine = QgsFeature(outLayer.pendingFields())
    (...)
    azimuth = fromPoint.azimuth(toPoint)
    newLine.setAttribute(AZIMUTH, azimuth)
    (...)

EDIT: I have added some timing print outputs. From print "time 1" to print "time 2" it takes approximately 4 seconds. The same with print "time 3" to print "time 4". This means one runthrough the outher for loop takes approximately 8 seconds (I know measuring the time with prints isn't the best way. But I just want to show the bottlenecks of the python code).

If you need some more information on my question, please feel free to leave a comment down below!

Answer 1

A few different ways to do this but if your lines are drawn in the same order as the points you can do this which avoids most of the API and just uses expressions:

features = iface.activeLayer().getFeatures()
exp = QgsExpression("azimuth(start_point($geometry), end_point($geometry))")

for f in features:
    result = exp.evaluate(f)
    print(result)
    ...
    newLine['azimuth'] = result