PyQGIS

For the Python and QGIS Workshop at HSR on 10th Mar 2017.

Introduction to PyQGIS

• QGIS supports scripting in Python
• Allows you to build plugins to extend the core functionality of QGIS and write scripts to automate tasks
• Python preferred over C++ plugins because of its simplicity of distribution and easier development

Setting up your development environment

To get started with PyQGIS, the following are required:

• QGIS 2.x (We will be using QGIS 2.18)
• Python 2.x (We will be using Python 2.7)
• Qt
• PyQt

Downloading the standalone or OSGeo4W installer will automatically install the correct version of Python as well as Qt and PyQt. (Link: Download QGIS)

Getting some sample data

• The PyQGIS Programmer's Guide's download page
• For raster layers, we can download one of the Natural Earth rasters
• In today's example we will use the Airports dataset and Populated places dataset retreived from Natural Earth

Using the QGIS Python Console

• With QGIS running, open the console using Plugins -> Python Console or by clicking on the Python console button on the Plugin toolbar
• The toolbar contains the tools Clear console, Import class, Run command, Show editor, Settings, and Help
• The built-in code editor can be used along with the console
• The QGIS API offers a large number of Python classes that we can use. See Searchable documentation of PyQGIS classes
• For the convenience of the user, the following statements are executed when the console is started

from qgis.core import *
import qgis.utils

Example

In this example we will load a vector point layer of the major airports in the world and write a Python script to print out the name, code, latitude and longitude of each airport in the northern hemisphere. To accomplish this, we:

Load the Vector Layer

We can load a vector layer in two ways:

1. Layer -> Add Layer -> Add Vector Layer -> Browse to the directory and select the shapefile
2. Use the Python console to open and display the vector layer

layer = iface.addVectorLayer('path_to_shapefile', 'layer_name', 'ogr')
if not layer:
    print "Layer failed to load"

Get a reference to the currently active layer

layer = iface.activeLayer()

Print out the name and iata_code of each airport feature

for f in layer.getFeatures():
    print f['name'], f['iata_code']

Get the geometry of each feature and print out its latitude (y) and longitude (x)

for f in layer.getFeatures():
    geom = f.geometry()
    print 'lat %.3f, lon %.3f' % (geom.asPoint().y(), geom.asPoint().x())

Python script to output to a text file the name, iata_code, latitude and longitude of each airport in the northern hemisphere

output_file = open('//svm-c113.hsr.ch/skhare/Desktop/PyQGIS/pyqgis_output.txt', 'w')
for f in layer.getFeatures():
  geom = f.geometry()
  # For all airports in the northern hemisphere, latitude (y coordinate) > 0
  if geom.asPoint().y() > 0:
    line = '%s, %s, %f, %f\n' % (f['name'], f['iata_code'],
          geom.asPoint().y(), geom.asPoint().x())
    unicode_line = line.encode('utf-8')
    output_file.write(unicode_line)
output_file.close()

Expressions in QGIS and Custom Python Expression Functions

Select using QGIS Expression

• Load the populated places shapefile
• Go to View -> Select -> Select by Expression
• Select all the features (populated places) lying within a country sov0name

Overview of Custom Python Expression Functions

• QGIS allows you to write user-defined functions in Python for selecting, calculating field values and labelling features
• Write Python code using the Function Editor and execute it in the Expression engine
• More examples can be found at QGIS Tutorials and Tips and User Defined Expression Function for QGIS

Resources and references

• The PyGIS Programmer's Guide - Gary Sherman
• PyQGIS Developer Cookbook - Official docs
• QGIS Tutorials