QgsSpatialIndex.nearestNeighbor() always returns 0 id

Question

I'm creating a set of features and adding them to a QgsSpatialIndex to find the ones closest to a certain location.

Why does nearestNeighbor() always return [0L] instead of the correct feature id?

import random 

index = QgsSpatialIndex()

for i in range(0,10):
    feat = QgsFeature()
    pt = QgsPoint(
        random.randint(0,10),
        random.randint(0,10))
    feat.setGeometry(QgsGeometry.fromPoint(pt))
    index.insertFeature(feat)

nearest = index.nearestNeighbor(QgsPoint(5,5),1)
print nearest 

Answer 1

Features only get an ID once they are added to a layer, but you can assign an ID manually using QgsFeature.setFeatureId(int) (in QGIS 2.18 and earlier) or QgsFeature.setId(int) (in newer versions) (Thanks to ndawson for the pointer!):

import random 

index = QgsSpatialIndex()

for i in range(0,10):
    feat = QgsFeature()
    pt = QgsPoint(
        random.randint(0,10),
        random.randint(0,10))
    feat.setGeometry(QgsGeometry.fromPoint(pt))
    feat.setFeatureId(int)
    index.insertFeature(feat)

nearest = index.nearestNeighbor(QgsPoint(5,5),1)
print nearest 

Alternatively, you can add the features to a memory layer:

import random 

index = QgsSpatialIndex()

vl = QgsVectorLayer("Point", "temporary_points", "memory")
pr = vl.dataProvider()

for i in range(0,10):
    feat = QgsFeature()
    pt = QgsPoint(
        random.randint(0,10),
        random.randint(0,10))
    feat.setGeometry(QgsGeometry.fromPoint(pt))
    pr.addFeatures([feat]) 

index = QgsSpatialIndex(vl.getFeatures())
nearest = index.nearestNeighbor(QgsPoint(5,5),1)
print nearest