How to find unique values of an attribute with python in qgis?

Question

i look for solution to get the unique values of an attribute of a vector layer. what is the best way in python qgis to get the unique values of one distinct attribute? I work with qgis 1.8.0.

Answer 1

QGIS provides a simple API to get this information. You need to get the index of the field in question and then query the layer for unique values.

idx = vectorLayer.fields().indexOf('fieldName')
values = vectorLayer.uniqueValues(idx)

Answer 2

Python's sets are handy for finding unique values. For example:

>>> set(range(4) + range(5) + range(6))
set([0, 1, 2, 3, 4, 5])

Try something like this (pyqgis 1.8):

values = set()
layer.select()
for feature in layer:
    index = layer.fieldNameIndex(ATTRNAME)
    values.add(feature.attributeMap()[index].toInt())

Pyqgis 2.0:

values = set()
for feature in layer.getFeatures():
    values.add(feature[ATTRNAME])

Answer 3

Late but maybe somebody can use it.

It is also posible to use the native python function. The uniqueValues function of the child for vector layers of the class QgsDataProvider.

See the Qgis API: QgsDataProvider and QgsVectorDataProvider

    uniquevalues = [];
    uniqueprovider = layer.dataProvider()
    fields = uniqueprovider.fields()
    id = fields.indexFromName('nameOfField')
    uniquevalues = uniqueprovider.uniqueValues( id )
    for uv in uniquevalues:
       print uv

i do that from qgis phyton console.