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 log files

Log files provide a way of tracking exactly what is going on in a Python plugin or script by creating messages that are available even if the script or QGIS crashes. These log messages make troubleshooting easier. In this recipe, we'll demonstrate two methods used for logging. One method is the actual log files on the filesystem, and the other is the QGIS Log Messages Panel window, which is available by clicking on the yellow triangle with an exclamation mark at the bottom-right corner of the QGIS application window, or by selecting the View menu, then clicking on Panels, and then checking Log Messages Panel.

Getting ready

To use log files, we must configure the QGIS_LOG_FILE environment variable by performing the following steps so that QGIS knows where to write log messages:

  1. From the QGIS Settings menu, select Options....

  2. In the Options dialog, select System panel.

  3. In the System panel, scroll down to the Environment section.

  4. In the Environment section, check the Use custom variables...