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

Storing and reading project preferences

The QGIS application settings are stored using the Qt API. However, QGIS project settings have their own object. In this recipe, we'll set and read the project title, and then set and read a custom preference for a plugin.

Getting ready

We are going to set a plugin preference using the sample plugin created in an earlier recipe, Creating a QGIS plugin, of this chapter. You can substitute the name of any plugin you want, however. We will also run this recipe in the QGIS Python console for quick testing, but this code will normally be used in a plugin.

How to do it...

In this recipe, we will first write and then read the title of the current project. Then, we will create a custom value for a plugin called splash, which can be used for the plugin startup splash screen if desired. Here are the steps to store and read project preferences:

  1. Start QGIS.

  2. From the Plugins menu, select Python Console.

  3. In the console, run the following code:

            proj = QgsProject.instance() 
            proj.title("My QGIS Project") 
            msg = "Geospatial Python Rocks!" 
            proj.writeEntry("MyPlugin", "splash", msg) 
            proj.readEntry("MyPlugin", "splash", "Welcome!")[0] 

How it works...

In the first two lines, we change the title of the current active project and then echo it back. In the next set of two lines, we set up and read the custom settings for a plugin. Notice that the readEntry() method returns a tuple with the desired text and a boolean, acknowledging that the value is set, so we extract the first index to get the text. The read method also allows the default text in case that property is not set (rather than throw an exception, which must be handled), as well as the boolean value False to inform you that the default text was used because the property was not set. The values you set using this method are stored in the project's XML file when you save it.

There's more...

The Qgs Project object has a number of methods and properties that may be useful. The QGIS API documentation details all of them at