This document outlines a workshop on Python programming in QGIS. It discusses loading and accessing vector and raster layers through Python, modifying layer attributes and geometries, and communicating with users. The workshop covers setting up Python in QGIS, loading layers, iterating over and assessing layers, modifying layers by starting edits and adding features, and visualizing maps on the canvas.

1. WORKSHOP ON THE
PYTHON PROGRAMMING
IN QGIS
Presented by:
Jyoti Dhakal (07)
Archana K.C (09)
Megha Shrestha (27)
Sushmita Timilsina (32)

2. Python programming and Qgis
• Python is a very powerful, simple and understandable
language
• QGIS is an open source for presenting, interpreting and
analyzing the geographic information

3. Program schedule
• Setup
• Loading layers in QGIS
• Assessing active layers
• Iterating over vector layers
• Modifying layers
• Communicating with User
• Using map canvas to visualize map

4. Setup
• QGIS is an open source
• For this workshop we are using QGIS 2.0.1 Dufor
• For python programming in QGIS
• Plugins -> Python console

5. •Before we begin
• from qgis.core import *
• import qgis.utils
Import the capabilities and utilities to be used through Python in
Python console

6. Loading vector layer through
python programming
• layer = QgsVectorLayer(data_source, layer_name, provider_name)
where:
QgsVectorLayer is the inbuilt class inside PyQgis
data_source is the location of the vector layer
layer_name is the name displayed in layers
provider_name is the name of the data provider

7. Contd…
• Layer from PostGIS database can be loaded using QgsDataSourceURI class
uri = QgsDataSourceURI()
uri.setConnection("localhost", "5432", "dbname", "johny", "xxx")
uri.setDataSource("public", "roads", "the_geom", "cityid = 2643")
vlayer = QgsVectorLayer(uri.uri(), "layer_name_you_like", "postgres")
• CSV or other delimited text files can be loaded as follows:
uri = "/some/path/file.csv?delimiter=%s&xField=%s&yField=%s" % (";", "x","y")
vlayer = QgsVectorLayer(uri, "layer_name_you_like", "delimitedtext")

8. Contd…
• GPX files
• SpatiaLite database
• WFS connection

9. Adding layer to the qgis map
layer
• QgsMapLayerRegistry is needed which is already defined in qgis.core
module
QgsMapLayerRegistry.instance().addMapLayer(layer)

10. Adding Raster Layer
• L=qgis.utils.iface.addRasterLayer('D:sushmitaCapture.gif')
Other information about the layer added
• L.width()
• L.height()
• L.extent()
• L.metadata()

11. Assessing the layers
• There are number of ways to assess the layers
• First of you need to import qgis.utils.iface
• Method 1 :
Run the following command:
>>> aLayer = qgis.utils.iface.activeLayer()
>>> aLayer
• Method 2 :
>>> canvas = qgis.utils.iface.mapCanvas()
>>> cLayer = canvas.currentLayer()
>>> cLayer.name()

12. • Method 3 :
>>> allLayers = canvas.layers()
>>> for i in allLayers: print i.name()...
It will return only the layers those are visible.
• Method 3 :
• It’s also useful sometimes to access layers in the order they are stacked in the table of
contents
>>> canvas.layer(0)
<qgis.core.QgsVectorLayer object at 0x99eaeec>
>>> canvas.layer(0).name()

13. Geometry type
• QGIS return 0,1,2 for Point, Line and Polygon features
respectively

14. Geometry construction
• from coordinates
gPnt = QgsGeometry.fromPoint(QgsPoint(1,1))
gLine = QgsGeometry.fromPolyline( [ QgsPoint(1,1), QgsPoint(2,2) ] )
gPolygon = QgsGeometry.fromPolygon( [ [ QgsPoint(1,1), QgsPoint(2,2),
QgsPoint(2,1) ] ] )
Accessor functions are required to extract the information
>>>gPnt.asPoint()
(1,1)
>>> gLine.asPolyline()
[(1,1), (2,2)]
>>> gPolygon.asPolygon()
[[(1,1), (2,2), (2,1), (1,1)]]

15. Iterating over vector layers
• features = layer.getFeatures()
• for f in features:
• geom = f.geometry()
• print "Feature ID %d: " % f.id()
• print "Area:", geom.area()
• print "Perimeter:", geom.length()
•

16. Modifying vector layers
• Start Editing
layer.startEditing()
• Functionality that supports editing of layer data
caps = layer.dataProvider().capabilities()
• Layer.commitChanges()
• Layer.rollback()

17. • # create layer
• vl = QgsVectorLayer("Point", "temporary_points", "memory")
• pr = vl.dataProvider()
• # add fields
• pr.addAttributes( [ QgsField("name", QVariant.String),
• QgsField("age", QVariant.Int),
• QgsField("size", QVariant.Double) ] )
• # add a feature
• fet = QgsFeature()
• fet.setGeometry( QgsGeometry.fromPoint(QgsPoint(10,10)) )
• fet.setAttributes(["Johny", 2, 0.3])
• pr.addFeatures([fet])

18. Communicating with the user
• Making a graphical user interface for easy communicating with the
user
• QgsMessageBar
• QPushButton
• progressMessageBar
• pushMessage

19. Map visualizing in canvas
• Map canvas is implemented as QgsMapCanvas class in qgis.gui
module