gistfile1.py

def change_values(layer, **kwargs):
    featuremap = {}
    for feature, values in kwargs.iteritems():
        attrs = {}
        for field, value in values.iteritems():
            index = layer.dataProvider().fieldNameIndex(field)
            attrs[index] = value
        featuremap[feature.id()] = attrs
    return layer.dataProvider().changeAttributeValues(featuremap)
    
features = {feature1: {"field1": 1, "field2": 2}, 
            feature2: {"field1": 1}} 
            
change_values(layer, features)