gistfile1.txt

import os
import sys
import sip

for api in ['QString', 'QDate', 'QDateTime', 'QTextStream',
            'QTime', 'QUrl', 'QVariant']:
    sip.setapi(api, 2)


from qgis.core import (
    QgsApplication, QgsVectorLayer, QgsField,
    QgsFields, QgsFeature, QgsGeometry, QgsPoint, QgsMapLayerRegistry)

from PyQt4.QtCore import QVariant


def main():
    # setup QGIS
    QgsApplication.setPrefixPath(os.environ['QGIS_PREFIX_PATH'], True)
    QgsApplication.initQgis()

    catalogue_layer = QgsVectorLayer(
        'Point?crs=epsg:4326', "catalogue", 'memory')

    QgsMapLayerRegistry.instance().addMapLayer(catalogue_layer)

    add_features(catalogue_layer)

    # print list(catalogue_layer.getFeatures())

    # print [f['cluster'] for f in catalogue_layer.getFeatures()]

    # edit features

    features = list(catalogue_layer.getFeatures())
    catalogue_layer.startEditing()
    # print features

    for f in features:
        f['cluster'] = 666.
        print f, f['eventID']
        catalogue_layer.changeAttributeValue(
            f['eventID'],
            f.fieldNameIndex('cluster'), 666.)

    catalogue_layer.commitChanges()
    print [(f['cluster'], f['eventID']) for f in features]
    print [(f['cluster'], f['eventID']) for f in catalogue_layer.getFeatures()]

    # Connect signal for app finish
    QgsApplication.exitQgis()
    sys.exit(0)


def add_features(vl):
    pr = vl.dataProvider()
    vl.startEditing()

    data = {
        'eventID': [0, 1, 2, 3, 4, 5],
        'latitude': [0, 1, 2, 3, 4, 5],
        'longitude': [0, 1, 2, 3, 4, 5],
        'cluster': [0, 1, 2, 3, 4, 5]
    }

    fields = []
    for key in data:
        fields.append(QgsField(key, QVariant.Double))
    pr.addAttributes(fields)

    features = []

    qgs_fields = QgsFields()
    for f in fields:
        qgs_fields.append(f)

    for i in range(6):
        fet = QgsFeature(i)
        fet.setFields(qgs_fields)

        x = data['longitude'][i]
        y = data['latitude'][i]
        fet.setGeometry(QgsGeometry.fromPoint(QgsPoint(x, y)))

        for key in data:
            event_data = data[key]
            fet[key] = event_data[i]
        features.append(fet)

    pr.addFeatures(features)
    vl.commitChanges()

    # print list(vl.getFeatures())


if __name__ == "__main__":
    main()

Why not

    for f in features:
        f['cluster'] = 666.
        print f, f['eventID']
        catalogue_layer.updateFeature(f)