PyQGIS Feature Selection – Selecting Multiple Features by ID-Attribute from List

featureslistpyqgisselect-by-expression

With PyQGIS I would like to use several "ID"s (in a list) to select certain objects in order to make them available later in a temporary layer. I know how to select one or a few more objects, but my list can contain a large number of "ID"s. My attempt to run

for x in list:
    layer.selectByExpression("id = '{}'".format(x))

in a for loop did not work. How can I select multiple features in a data set?

Best Answer

Use this way (without for loop):

id_list = [1, 2, 5]
layer.selectByExpression("id in {}".format(tuple(id_list)))

Related Solutions

PyQGIS – Subsetting a Shapefile and Saving It

I guess there is an easier way to select your layer, subset your data and save it :

lyr = QgsMapLayerRegistry.instance().mapLayersByName("Layer_name")[0]
lyr.setSubsetString("stat_i != 0") # You can unset it by using None 
QgsVectorFileWriter.writeAsVectorFormat(lyr, "my_subset.shp",
                                        "UTF8", None, "ESRI Shapefile")

And so on with the n shapefiles you have to write.

Edit : I missed up the part where you want to select specific fields. I guess you can try (and improve!) something like this :

lyr = QgsMapLayerRegistry.instance().mapLayersByName("Layer_name")[0]
# Lets say you have 10 fields 'stat_x':
for i in range(10):
    lyr.setSubsetString("stat_{} != 0".format(i))
    fields = QgsFields() # It will contain your new fields (name and type)
    field1, field2 = 'id', 'stat_{}'.format(i)
    [fields.append(fi) for fi
     in (QgsField(field1, QVariant.Int),QgsField(field2, QVariant.Int))]

    # Here the args for QgsVectorFileWriter will be : 
    # FileName, Encoding, fields, GeometryType, CRS
    # See also http://qgis.org/api/classQgsVectorFileWriter.html
    writer_obj = QgsVectorFileWriter(
        "my_subset_n{}.shp".format(i), None, fields,
        lyr.dataProvider().geometryType(), lyr.crs())

    for ft in lyr.getFeatures():  # Loop on the subseted features
        # Create a new feature for each original feature
        new_ft = QgsFeature()
        # Give it its geometry :
        new_ft.setGeometry(ft.geometry())
        # Only keep the 2 selected fields :
        new_ft.setAttributes([ft[field1], ft[field2]])
        # Add the feature to the writer, ie. your output shapefile :
        writer_obj.addFeature(fet)

    del writer_obj # Features are written when the writer is deleted

If you haven't seen it, there is some useful insights in the pyQGIS doc about vector layers

Best Answer

Related Solutions

PyQGIS – Subsetting a Shapefile and Saving It

Related Question