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Building Mapping Applications with QGIS
Building Mapping Applications with QGIS

Building Mapping Applications with QGIS: Create your own sophisticated applications to analyze and display geospatial information using QGIS and Python

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Building Mapping Applications with QGIS

Chapter 2. The QGIS Python Console

In this chapter, we will look at the ways in which you can use the QGIS Python Console as a geospatial development tool. We will also use the console as a looking-glass to examine the world of QGIS programming. In particular, we will learn the following:

  • Explore the ways in which the console can be used to develop and execute Python code
  • Learn how to write Python scripts using the console's built-in source code editor
  • Discover various tips and techniques to work with the QGIS Console
  • Figure out how to manipulate the current project within QGIS using Python commands
  • Access geospatial data and perform geospatial calculations using the console
  • Use various QGIS user-interface elements within our Python programs

Using the console

While you briefly used the QGIS Console in the previous chapter, it is worth examining the QGIS Console window in more detail, so that you are aware of the various features that are available.

If you don't already have it open, choose the Python Console item from the Plugins menu to open the console. The following screenshot shows the various parts of the console window:

Using the console

Let's take a closer look at these various parts:

  • The Clear console button wipes out the contents of the interpreter log
  • The Import Class pop up contains shortcuts to import some commonly-used PyQGIS classes
    Using the console

    These are equivalent to typing import Processing, from PyQt4.QtCore import *, and from PyQt4.QtGui import *.

  • The Run command button simply executes the command you have typed in the Python shell field

    Note

    Of course, you can also run the entered command by pressing the Return key, so this command is only useful if you really want to run a command using the mouse.

  • The Show editor button shows or hides...

Working with geospatial data in the console

So far, we have used the QGIS Console as a glorified Python interpreter, running standard Python programs and manipulating the QGIS user interface. But QGIS is a Geographical Information System (GIS), and one of the main uses of a GIS is to manipulate and query geospatial data. So, let's write some Python code to work with geospatial data directly within the QGIS Console.

In the previous chapter, we loaded three shapefiles into a QGIS project using Python. Here is a typical instruction we used to load a shapefile into a QGIS map layer:

layer = iface.addVectorLayer("/path/to/shapefile.shp", "layer_name", "ogr")

While this is useful if you want to create a QGIS project programmatically, you may just want to load a shapefile so you can analyze its contents, without putting the data into a map layer. To do this, we have to get an appropriate data provider and ask it to open the shapefile, like this:

registry = QgsProviderRegistry...

Scripting the QGIS user interface

While the example program we created earlier has very limited user interaction, it is quite possible to build your program to directly use the QGIS user interface elements such as the status bar, the message bar, progress indicators, and the QGIS logging window. You can also create custom forms and windows so that the output of your program looks just like any other feature of QGIS itself. Let's take a closer look at how some of these QGIS user-interface elements can be used from within your Python programs.

The status bar

The QGIS window has a status bar. You can use it to display the current status of your Python program, for example:

iface.mainWindow().statusBar().showMessage("Please wait...")

The status message will appear at the bottom of the window, like this:

The status bar

As you can see, there isn't much room on the status bar, so you'll need to keep your status message short. To hide the message again, do the following:

iface.mainWindow()...

Summary

In this chapter, we explored the QGIS Python Console, and how to use it for a variety of programming tasks. We also used the console to delve more deeply into the QGIS Python programming environment.

As we worked through this chapter, we learned what the various toolbar buttons and controls do within the QGIS Console, and how to enter commands using the Python Shell. We looked at how we can use the Python Interpreter Log to view the previous output and re-enter commands you executed earlier. We saw the ways in which you can use autocompletion to enter your Python code more quickly, and also learned about the parameters that the various PyQGIS functions and methods accept.

We then looked at how to enter and execute Python scripts using the built-in source code editor. We discovered that the Python Console is itself written in Python, allowing you to explore the source code and manipulate the console itself using the Python code.

We learned how to create a startup script that is run...

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Description

If you are an experienced Python developer who wants to create your own geospatial applications with minimum fuss, this is the book for you. While some familiarity with mapping applications would be an advantage, no prior knowledge of geospatial concepts is required. Even if you've never used QGIS before, this book will quickly get you up to speed.

What you will learn

  • Use the Python Console to explore and control QGIS
  • Perform geospatial development tasks using the PyQGIS Python library
  • Create useful geospatial applications implemented as QGIS Python plugins
  • Use QGIS as a standalone geoprocessing library within your Python programs
  • Build complex interactive mapbased GUIs within your own Python programs
  • Understand how Python scripting can be used to customize the QGIS system
  • Utilize the true potential of Python and QGIS to create your own complete mapping applications
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Publication date : Dec 30, 2014
Length: 264 pages
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Language : English
ISBN-13 : 9781783984664
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Table of Contents

10 Chapters
1. Getting Started with QGIS Chevron down icon Chevron up icon
2. The QGIS Python Console Chevron down icon Chevron up icon
3. Learning the QGIS Python API Chevron down icon Chevron up icon
4. Creating QGIS Plugins Chevron down icon Chevron up icon
5. Using QGIS in an External Application Chevron down icon Chevron up icon
6. Mastering the QGIS Python API Chevron down icon Chevron up icon
7. Selecting and Editing Features in a PyQGIS Application Chevron down icon Chevron up icon
8. Building a Complete Mapping Application using Python and QGIS Chevron down icon Chevron up icon
9. Completing the ForestTrails Application Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

Customer reviews

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Full star icon Full star icon Full star icon Full star icon Half star icon 4.6
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Nyall Apr 20, 2015
Full star icon Full star icon Full star icon Full star icon Full star icon 5
I’m a huge fan of this work and think it may be my favourite QGIS book to date! I’ve read Erik’s previous work, Python Geospatial Development, and thought it was an entertaining and really well written book. He’s clearly got an in-depth knowledge about what he’s writing about and this confidence comes through in his writing.In Building Mapping Applications with QGIS, Erik has created a comprehensive guide through all the steps required to create QGIS plugins and standalone Python applications which utilise the QGIS libraries. It’s not a beginner’s guide to Python or to PyQGIS, but that’s what helps it stand out. There’s no introductory chapters on programming with Python or how to use QGIS and instead Erik dives straight into the meat of this topic. I found this approach really refreshing, as I’m often frustrated when the first few chapters of an advanced work just cover the basics. Instead, Building Mapping Applications with QGIS is packed with lessons about, well, actually building mapping applications!So, why do I like this book so much? Personally, I think it fills a a really crucial void in the existing QGIS literature. There’s a lot of works covering using QGIS, and a few covering PyQGIS development (eg, the PyQGIS Programmer’s Guide, which I reviewed here). But to date, there hasn’t been any literature that covers developing QGIS based applications in such great depth. It’s just icing on the cake that Erik’s writing is also so interesting and easy to read.
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willian alves Feb 13, 2015
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Excelente para quem já sabe programar em python e pretende explorar as funcionalidades que as bibliotecas do QGIS podem oferecer. O livro é bem estruturado e trás exemplos deste como usar simples comandos até fazer plugins e programas completos independentes.
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Sukhoon Hyun May 11, 2015
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Good!!
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ainardi Sep 29, 2015
Full star icon Full star icon Full star icon Full star icon Full star icon 5
you want to learn more about qgisthis book is full of good informationyou want to learn more on how to build application, this book is for you
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Amazon Customer Feb 06, 2016
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Awesome book! Helped me a lot for my Master thesis!
Amazon Verified review Amazon
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