This recipe performs the simplest possible edit to a vector layer instantiated from a shapefile. We will add a point to an existing point layer.
For this recipe, download the zipped shapefile from https://geospatialpython.googlecode.com/svn/NYC_MUSEUMS_GEO.zip.
Extract the .shp, .shx, and .dbf files to the /qgis_data/nyc directory.
We will load the vector layer from the shapefile, create a new geometry object as a point, create a new feature, set the geometry, and add it to the layer's data provider. Finally, we will update the extent of the layer to make sure that the bounding box of the layer encapsulates the new point:
- Start QGIS.
- From the Plugins menu, select Python Console.
- First, load the layer:
vectorLyr = QgsVectorLayer('/qgis_data/nyc/NYC_MUSEUMS_GEO.shp', 'Museums' , "ogr") - Now, will access the layer's data provider:
vpr = vectorLyr.dataProvider() - Next, create a new point using the
QgsGeometryobject:pnt = QgsGeometry.fromPoint(QgsPoint(-74.80,40.549)) - Now, will create a new
QgsFeatureobject to house the geometry:f = QgsFeature() - Next, set the geometry of the feature using our point:
f.setGeometry(pnt) - Then, place the features into the layer's feature list:
vpr.addFeatures([f]) - Finally, update the layer's extent to complete the addition:
vectorLyr.updateExtents()
PyQGIS abstracts the points within a layer into four levels. At the lowest level is the QgsPoint object, which contains nothing more than the coordinates of the point. This object is added to an abstract QgsGeometry object. This object becomes the geometric part of a QgsFeature object, which also has the ability to store and manage attributes. All the features are managed by the QgsDataProvider object. The data provider manages the geospatial aspect of a layer to separate that aspect from styling and other presentation-related portions. QGIS has another editing approach in Python, which is called an
editing buffer. When you use an editing buffer, the changes can be displayed, but they are not permanent until you commit them. The most common use case for this editing method is in GUI applications where the user may decide to roll back the changes by cancelling the editing session. The PyQGIS Developer Cookbook has an example of using and editing buffers in Python, and is available at http://docs.qgis.org/2.6/en/docs/pyqgis_developer_cookbook/vector.html.