Book Image

QGIS Python Programming Cookbook, Second Edition - Second Edition

By : Joel Lawhead
Book Image

QGIS Python Programming Cookbook, Second Edition - Second Edition

By: Joel Lawhead

Overview of this book

QGIS is a desktop geographic information system that facilitates data viewing, editing, and analysis. Paired with the most efficient scripting language—Python, we can write effective scripts that extend the core functionality of QGIS. Based on version QGIS 2.18, this book will teach you how to write Python code that works with spatial data to automate geoprocessing tasks in QGIS. It will cover topics such as querying and editing vector data and using raster data. You will also learn to create, edit, and optimize a vector layer for faster queries, reproject a vector layer, reduce the number of vertices in a vector layer without losing critical data, and convert a raster to a vector. Following this, you will work through recipes that will help you compose static maps, create heavily customized maps, and add specialized labels and annotations. As well as this, we’ll also share a few tips and tricks based on different aspects of QGIS.
Table of Contents (16 chapters)
QGIS Python Programming Cookbook - Second Edition
About the Author
About the Reviewer
Customer Feedback

Using NULL values in PyQGIS

QGIS can use NULL values as field values. Python uses None objects of type None instead of NULL values, which are null memory references. The concepts are similar. You must be aware of this fact when working with Python in QGIS, which is a mixture of C++ and Python. In this recipe, we'll explore the implications of QGIS NULL values in Python.

Getting ready

In your qgis_data/shapes directory, download the following shapefile, which contains some NULL field values, and unzip it:

How to do it...

We will load the shapefile and grab its first feature. Then, we'll access one of its NULL field values. Next, we'll run through some tests that allow you to see how the NULL values behave in Python:

  1. Load the shapefile and access its first feature:

            lyrPth = "/qgis_data/shapes/NullExample.shp" 
            lyr = QgsVectorLayer(lyrPth, "Null Field Example", "ogr") 
            features = lyr.getFeatures...