Adding a field to a vector layer

This recipe demonstrates adding a new field to a layer. Each field represents a new column in a data set for which each feature has a new attribute. When you add a new attribute, the attribute value for all existing features are set to NULL for that field index.

Getting ready

We will use the New York City museums shapefile used in other recipes, which you can download as a ZIP file here:

https://github.com/GeospatialPython/Learn/raw/master/NYC_MUSEUMS_GEO.zip

Extract this shapefile to /qgis_data/nyc.

How to do it...

All data management for a layer is handled through the layer's data provider, and fields are no different. We will load the layer, access the data provider, define the new field, and finalize the change:

1. Start QGIS.
2. From the Plugins menu, select Python Console.
3. First, we must import the Qt library data types that PyQGIS uses to specify layer field data types:

           from PyQt4.QtCore import QVariant 
   

4. Next, load and validate the layer:

           vectorLyr=  QgsVectorLayer('/qgis_data/nyc/NYC_MUSEUMS_GEO.shp',
                                      'Museums' , "ogr") 
           vectorLyr.isValid() 
   

5. Then, we access the layer data provider:

           vpr = vectorLyr.dataProvider() 
   

6. Now, we add a Python list of QgsField objects, which define the field name and type. In this case, we'll add one field named Admission as a double:

           vpr.addAttributes([QgsField("Admission", QVariant.Double)]) 
   

7. Finally, we update the fields to complete the change:

           vectorLyr.updateFields()  
   

How it works...

It is important to note that QGIS refers to the column headers as attributes and the feature values as fields. In other GIS packages, you may see this nomenclature reversed.