Using the QGIS Python console for interactive control

The Python console allows you to interactively control QGIS; you can test out ideas or just do some quick automation. The console is the simplest way to use the API.

How to do it...

In the following steps, we'll open the console, create a vector layer in the memory, and display it on the map:

1. Start QGIS.
2. From the Plugins menu, select Python Console.
3. The following code will create a point on the map canvas:

           layer =  QgsVectorLayer('Point?crs=epsg:4326','MyPoint',"memory") 
           pr = layer.dataProvider() 
           pt = QgsFeature() 
           point1 = QgsPoint(20,20) 
           pt.setGeometry(QgsGeometry.fromPoint(point1)) 
           pr.addFeatures([pt]) 
           layer.updateExtents() 
           QgsMapLayerRegistry.instance().addMapLayers([layer]) 
   

How it works...

This example uses a memory layer to avoid interacting with any data on either a disk or a network so as to keep things simple. Notice that when we declare the layer type, we add the parameter for the coordinate reference system (CRS) as EPSG:4326. Without this declaration, QGIS will prompt you to choose one. There are three parts, or levels, of abstraction even to create a single point on the map canvas, as shown here:

1. First, create a layer that is of the type geometry. Next, reference a data provider to accept the data source.
2. Then, create a generic feature object, followed by the point geometry.
3. Next, we add the feature to the layer via the underlying data provider, then add the layer to the map canvas.

The layer type is memory, meaning that you can define the geometry and attributes in the code itself rather than an external data source. In this recipe, we just define the geometry and skip defining any attributes.