Speeding up length calculation in QGIS 2.0 Python plugin

Question

In a qgis 2.0 plugin for windows, I calculate length and update a field with the values. It works, but takes a lot of time. Execution for this part only takes about ten minutes. Can anyone suggest anything to speed things up?

def calculatelength(self):
    idx = self.resLayer.fieldNameIndex('Longueur')
    caps = self.resLayer.dataProvider().capabilities()
    features = self.resLayer.getFeatures()
    for feature in features:
        if caps & QgsVectorDataProvider.ChangeAttributeValues:
            fid = feature.id()
            flen = feature.geometry().length()
            attrs = {idx : flen}
            self.resLayer.dataProvider().changeAttributeValues({fid : attrs})

Answer 1

I think the call to resLayer.dataProvider().changeAttributeValues has too much overhead in terms of disk access+writing, etc.. Try storing all your edits in a dictionary(or list) and then doing the editing in edit mode all in one go.

def calculatelength(self):
    idx = self.resLayer.fieldNameIndex('Longueur')
    caps = self.resLayer.dataProvider().capabilities()
    features = self.resLayer.getFeatures()
    edits = {}
    for feature in features:
        if caps & QgsVectorDataProvider.ChangeAttributeValues:
            fid = feature.id()
            flen = feature.geometry().length()
            edits[fid] = flen

    self.resLayer.startEditing()
    for fid in edits:
        self.resLayer.changeAttributeValue(fid, idx, edits[fid])
    self.resLayer.commitChanges()