Each QGIS feature has two parts, the geometry and the attributes. In this recipe, we'll add an attribute for a layer from an existing dataset.
We will use a point shapefile with museum data for New York City, which you can download as a ZIP file from https://geospatialpython.googlecode.com/svn/NYC_MUSEUMS_GEO.zip.
Extract this shapefile to the /qgis_data/nyc directory.
A feature must have geometry, but it does not require attributes. So, we will create a new feature, add some attributes, and then add everything to the layer, as follows:
- Start QGIS.
- From the Plugins menu, select Python Console.
- First, load the layer and validate it:
vectorLyr = QgsVectorLayer('/qgis_data/nyc/NYC_MUSEUMS_GEO.shp', 'Museums' , "ogr") vectorLyr.isValid()
- Next, access the layer's data provider so that we can get the list of fields:
vpr = vectorLyr.dataProvider() - Now, create a point geometry, which in this case is a new museum:
pnt = QgsGeometry.fromPoint(QgsPoint(-74.13401,40.62148)) - Next, get the
fieldsobject for the layer that we'll need to create a new feature for:fields = vpr.fields() - Then, create a new feature and initialize the attributes:
f = QgsFeature(fields) - Now, set the geometry of our new museum feature:
f.setGeometry(pnt) - Now, we are able to add a new attribute. Adding an attribute is similar to updating a Python dictionary, as shown here:
f['NAME'] = 'Python Museum' - Finally, we add the feature to the layer and update the extents:
vpr.addFeatures([f]) vectorLyr.updateExtents()
PyQGIS attributes are defined as an ordered array. The syntax for referencing a field is similar to the syntax for a Python dictionary. We use the layer's data provider object to perform the actual editing. When we use this approach, no signals are triggered at the layer object level. If we are just trying to edit data on the filesystem, that's okay, but if the layer is going to be added to the map canvas for display or user interaction, then you should use the editing buffer in the QgsVectorLayer object. This editing buffer allows you to commit or roll back changes and also keeps track of the state of the layer when things are changed.