QGIS Python Index – How to Generate an Index Field in QGIS Using Python for Optimized Data Management

fields-attributespyqgisqgis-2.18

I have some shapefiles for which I need to generate an index(int) field.

I tried using this function:

def add_index_to_layer(layer):
    fieldindex = layer.fieldNameIndex("index")
    if fieldindex >= 0:
        print "Layer " + layer.name() + " already has an index field"
        return False
    layer.dataProvider().addAttributes([QgsField("index", QVariant.Int)])
    layer.updateFields() # no idea if this is actually needed?
    fieldindex = layer.fieldNameIndex("index")
    i = 0
    layer.startEditing()
    for field in layer.fields():
        print field.name() # this shows the field "index" indeed exists
    for feature in layer.getFeatures():
        print feature.id(), feature["index"]
        feature.setAttribute("index", i)
        # this shows the value is being correctly set at this point
        print feature.id(), feature["index"] 
        i +=1
    layer.commitChanges()

The function runs just fine, the various print functions also show everything works as expected (values being set correctly).

However, when verifiyng by looking at the attribute table, the index field is created, but it always is empty/NULL.

Does anyone have an idea about what is going on? Are some of the changes pending where I expect them to be commited already? I thought layer.startEditing() and layer.commitChanges() would properly encapsulate feature field changes?

I am working with QGIS 2.18.0, and the script is created as a processing script.

Best Answer

Try replacing this line:

feature.setAttribute("index", i)

with this:

layer.changeAttributeValue(feature.id(), layer.fieldNameIndex("index"), i)

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pyqgis – Adding Fields to Layer in QGIS Using Python Plugin

One method (which you are currently using) is the standard QComboBox. First we need to import some more classes so add this before importing PyQt4:

from qgis.core import QgsMapLayerRegistry, QgsField, QgsExpression
from PyQt4.QtCore import QVariant, QSettings, QTranslator, qVersion, QCoreApplication

Then in your run(self) function, we can use the following (I've added some comments which hopefully will help you understand what's going on):

def run(self):
    # here I populate a combo box in a form
    layers = self.iface.legendInterface().layers()
    layer_list = []
    for layer in layers:
        layer_list.append(layer.name())
    self.dlg.Cmb_Capa.addItems(layer_list)

    self.dlg.show()
    result = self.dlg.exec_()
    if result:
        # Read current text from combo box
        combo_layer = self.dlg.Cmb_Capa.currentText()
        # Identify layer from table of contents by matching name with the combo box text
        selected_layer = QgsMapLayerRegistry.instance().mapLayersByName(combo_layer)[0]
        # Add a "real" field type with the name "Area"
        selected_layer.dataProvider().addAttributes( [ QgsField("Area", QVariant.Double) ] )
        # Update the attribute table
        selected_layer.updateFields()
        # Allow editing of the selected layer
        selected_layer.startEditing()
        # Find index of the newly-created "Area" field
        idx = selected_layer.fieldNameIndex( "Area" )
        # Use standard QgsExpression to calculate area for each feature
        e = QgsExpression( """ $area """ )
        e.prepare( selected_layer.pendingFields() )
        for f in selected_layer.getFeatures():
            f[idx] = e.evaluate(f)
            selected_layer.updateFeature(f)
        # Save the selected layer
        selected_layer.commitChanges()

Alternatively, you could use QGIS custom widget: QgsMapLayerComboBox. This is automatically updated when any changes are made to the layer list (i.e. layers added/removed, names changed etc) without you having to refresh the plugin. To use this, you will need to import an additional class:

from qgis.gui import QgsMapLayerProxyModel

Now your run(self) function can look like this:

 def run(self):
    # Here, QgsMapLayerComboBox automatically reads the relevant layers
    self.dlg.Cmb_Capa.setFilters( QgsMapLayerProxyModel.PolygonLayer )

    self.dlg.show()
    result = self.dlg.exec_()
    if result:
        combo_layer = self.dlg.Cmb_Capa.currentText()
        selected_layer = QgsMapLayerRegistry.instance().mapLayersByName(combo_layer)[0]
        selected_layer.dataProvider().addAttributes( [ QgsField("Area", QVariant.Double) ] )
        selected_layer.updateFields()
        selected_layer.startEditing()
        idx = selected_layer.fieldNameIndex( "Area" )
        e = QgsExpression( """ $area """ )
        e.prepare( selected_layer.pendingFields() )
        for f in selected_layer.getFeatures():
            f[idx] = e.evaluate(f)
            selected_layer.updateFeature(f)
        selected_layer.commitChanges()

Note: When using QGIS custom widgets, you may need to edit your "plugin_name_dialog_base.ui" file, find the QgsMapLayerComboBox class and replace the <header> to qgis.gui.h.

For example, it might look like this:

<customwidget>
 <class>QgsMapLayerComboBox</class>
 <extends>QComboBox</extends>
 <header>qgsmaplayercombobox.h</header>
</customwidget>

And it needs to look like this:

<customwidget>
 <class>QgsMapLayerComboBox</class>
 <extends>QComboBox</extends>
 <header>qgis.gui.h</header>
</customwidget>

Best Answer

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