Finding unique values of attribute with PyQGIS

Question

I am looking for solution to get the unique values of an attribute of a vector layer.

What is the best way in PyQGIS to get the unique values of one distinct attribute?

I have polygons described by an attribute "group". Each group contains 1 to about 10 polygons. I want to process the polygons belonging to a certain group in for-loop, i.e. for each iteration of the for-loop i select the polygons with setSubsetString to process this group, e.g. to write them to a shape file. However I do not know the group before reading the vector layer.

I work with QGIS 1.8.0.

• underdark: There is a split tool which will create separate shapefiles depending on an attribute if that's all you need. Otherwise, that plugin would also be the place to look for sample code.
• gallay: From the plugin you were mentioned the function split in splitterdialog.py is what i was looking for. Thank you for the hint!

Answer 1

QGIS provides a method .uniqueValues(fieldIndex) of the QgsVectorLayer class to get this information. You need to get the index of the field in question and then query the layer for unique values.

vectorLayer = iface.activeLayer()
idx = vectorLayer.fields().indexOf('FIELD_NAME')
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 2.0:

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

PyQGIS 1.8:

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

Answer 3

There is a QGIS algorithm for that called "List unique values"

layer = iface.activeLayer()
params = {
 'INPUT' : layer,
 'FIELDS' : ['FIELD_NAME'],
 'OUTPUT' : 'TEMPORARY_OUTPUT',
 'OUTPUT_HTML_FILE' : 'TEMPORARY_OUTPUT'
 }
result = processing.run("qgis:listuniquevalues", params)
print(result['UNIQUE_VALUES'])

• Taras: Dear @saheka, I decided to rollback your answer, because I noticed, that your initial version was unique (the last edit version overlapped too much with MatthiasKuhn's answer) and it is still usable.