Add data to attribute problem in PyQGIS?

Question

Based on xunilk code in previous post I improved it to add mean distances to "MeanDist" field, while I can print mean distance for each point id, but this data didn't write in attribute of layer. what is the problem for add data to the attribute?

from PyQt4.QtCore import *
import itertools
import numpy as np

layer = iface.activeLayer()

feats = [ feat for feat in layer.getFeatures() ]

n = len(feats)

comb = range(n)

distances = [ [] for i in range(n) ]
indexes = [ [] for i in range(n) ]

for i, j in itertools.combinations(comb, 2):

    dist = feats[i].geometry().distance(feats[j].geometry())

    if dist < 500:
        i_dist= distances[i].append(dist)
        i_index= indexes[i].append([i,j])
        j_dist= distances[j].append(dist)
        j_index= indexes[j].append([i,j])

prov = layer.dataProvider()
layer.startEditing()

if prov.fieldNameIndex("MeanDist") == -1:
    prov.addAttributes([QgsField("MeanDist", QVariant.Double, "double",    10, 2)])
else:
    pass

for feature in layer.getFeatures():
    attrName = "MeanDist"
    for i, group in enumerate(distances):
        if i == feature.id():
            print feature.id(), np.mean(group)   # print for test results, it must remove.
            prov.changeAttributeValues({feature.id() : {prov.fieldNameMap()[attrName]: np.mean(group)}})
            layer.updateFeature(feature)

layer.updateFields()
layer.commitChanges()

Answer 1

I see several issues:

1. Include .updateFields() after adding the attribute
2. .fieldNameIndex() should be done on the layer not the provider
3. .changeAttributeValues() should be done on the layer not the provider
4. .updateFields() should be done on the layer not the provider
5. Start edit session after .updateFields() method
6. You do not need layer.updateFeature(feature)

Try:

if layer.fieldNameIndex("MeanDist") == -1:
    prov.addAttributes([QgsField("MeanDist", QVariant.Double, "double",    10, 2)])
    layer.updateFields()
else:
    pass

layer.startEditing()
for feature in layer.getFeatures():
    attrName = "MeanDist"
    for i, group in enumerate(distances):
        if i == feature.id():
            print feature.id(), np.mean(group)   # print for test results, it must remove.
            layer.changeAttributeValue(feature.id(), layer.fieldNameIndex(attrName), np.mean(group))

layer.commitChanges()