Qgis Python Programming Cookbook Pdf

[Lotte Donohoe]

Thischapter explains how to configure QGIS for automation using Python. In addition to setting up QGIS, we will also configure the free Eclipse Integrated Development Environment (IDE) with the PyDev plugin to make writing, editing, and debugging scripts easier. We will also learn the basics of different types of QGIS automated Python scripts through the PyQGIS API. Finally, we'll examine some core QGIS plugins that significantly extend the capability of QGIS.

QGIS has a set of Python modules and libraries that can be accessed from the Python console within QGIS. However, they can also be accessed from outside QGIS to write standalone applications. First, you must make sure that PyQGIS is installed for your platform, and then set up some required system environment variables.

In this recipe, we will walk you through the additional steps required beyond the normal QGIS installation to prepare your system for development. The steps for each platform are provided, which also include the different styles of Linux package managers.

However, on Linux distributions and Mac OS X, you may need to manually install the Python modules for the system installation of Python. On Mac OS X, you can download installers for some of the commonly used Python modules with QGIS from

The QGIS installation process and package managers set up the Python module's configuration internal to QGIS. When you use the Python console inside QGIS, it knows where all the PyQGIS modules are. However, if you want to use the PyQGIS API outside QGIS, using a system Python installation on Windows or Linux, it is necessary to set some system variables so that Python can find the required PyQGIS modules.

This example uses a memory layer to avoid interacting with any data on disk or a network to keep things simple. Notice that when we declare the layer type, we add the parameter for the Coordinate Reference System (CRS) as EPSG:4326. Without this declaration, QGIS will prompt you to choose one. There are three parts or levels of abstraction to create even a single point on the map canvas, as shown here:

The layer type is memory, meaning that you can define the geometry and the attributes inline in the code rather than in an external data source. In this recipe, we just define the geometry and skip the defining of any attributes.

The QGIS Python ScriptRunner plugin provides a middle ground for QGIS automation, between the interactive console and the overhead of plugins. It provides a script management dialog that allows you to easily load, create, edit, and run scripts for large-scale QGIS automation.

Install the ScriptRunner plugin using the QGIS plugin manager. Then, run the plugin from the Plugin menu to open the ScriptRunner dialog. Configure a default editor to edit scripts using the following steps:

ScriptRunner is a simple but powerful idea. It allows you to build a library of automation scripts and use them from within QGIS, but without the overhead of building a plugin or a standalone application. All the Python and system path variables are set correctly and inherited from QGIS; however, you must still import the QGIS and Qt libraries.

This tool makes an excellent PyQGIS IDE. Eclipse allows you to have multiple Python interpreters configured for different Python environments. When you install PyDev, it automatically finds the installed system Python installations. On Windows, you must also add the Python interpreter installed with PyQGIS. On all platforms, you must tell PyDev where the PyQGIS libraries are.

This recipe uses Eclipse and PyDev. You can use the latest version of either package that is supported by your operating system. All platforms besides Windows rely on the system Python interpreter. So, there is an extra step in Windows to add the QGIS Python interpreter.

The process used to add the QGIS Python interpreter to Eclipse on Windows is different from the process used on Linux. The following steps describe how to set up the interpreter on the Windows version of Eclipse:

Browse to the location of the QGIS Python interpreter called python.exe within the bin folder of the QGIS program folder. On OS X and Linux, you use can use the system Python installation. On Windows, Python is included with QGIS. The default location on Windows is C:\Program Files\QGIS Brighton\bin\python.exe, as shown in the following screenshot:

When you click on the OK button, Eclipse will attempt to automatically add every Python library it finds to the Python path for this interpreter configuration. We need to control which libraries are added to prevent conflicts. Click on the Deselect All button and then click on OK:

Apart from adding the Python interpreter, you must also add the module paths needed by PyQGIS using the following steps. These steps will require you to switch back and forth between QGIS and Eclipse:

For each path in the returned lists, click on the New Folder button in Eclipse's Libraries pane for your QGIS interpreter, and browse to that folder until all the paths have been added. If a given folder does not exist on your system, simply ignore it, as shown here:

To take full advantage of Eclipse's features, including code completion, we will add the QGIS and Qt4 modules to the PyQGIS Eclipse interpreter preferences. The following steps will allow Eclipse to suggest the possible methods and properties of QGIS objects as you type; this feature is known as autocomplete:

For the Value field, add the path to the QGIS program directory and to any QGIS directories containing binaries separated by a semicolon. The following is an example from a Windows machine:

Eclipse and PyDev use only the information you provide to run a script in the Eclipse workspace. This approach is very similar to the popular Python tool virtualenv, which provides a clean environment when writing and debugging code to ensure that you don't waste time troubleshooting issues caused by the environment.

Both QGIS and Eclipse must be configured for debugging so that the two pieces of software can communicate. Eclipse attaches itself to QGIS in order to give you insights into the Python scripts running in QGIS. This approach allows you to run scripts in a controlled way that can pause execution while you monitor the program to catch bugs as they occur.

The following steps will add two plugins to QGIS, which allows Eclipse to communicate with QGIS. One plugin, Plugin Reloader, allows you to reload a QGIS plugin into memory without restarting QGIS for faster testing. The second plugin, Remote Debug, connects QGIS to Eclipse.

The previous parts of this recipe configured Eclipse and QGIS to work together in order to debug QGIS plugins. In this section, we will test the configuration using the simplest possible plugin, HelloWorld, to run Eclipse using the Debug Perspective. We will set up a break point in the plugin to pause the execution and then monitor plugin execution from within Eclipse, as follows:

The RemoteDebug plugin acts as a client to the PyDev debug server in order to send the Python script's execution status from QGIS to Eclipse. While it has been around for several versions of QGIS now, it is still considered experimental.

The PluginReloader plugin can reset plugins that maintain state as they run. The HelloWorld plugin is so simple that reloading is not needed to test it repeatedly. However, as you debug more complex plugins, you will need to run it in order to reset it before each test. This method is far more efficient and easier to use than closing QGIS, editing the plugin code, and then restarting.

The QGIS Python API, also known as PyQGIS, allows you to control virtually every aspect of QGIS. The ability to find the PyQGIS object you need in order to access a particular feature of QGIS is critical to automation.

The PyQGIS API documentation is not updated frequently because it is nearly identical to the structure of the C++ API. However, the QGIS project on github.com maintains a list of all the PyQGIS classes for the latest version. The PyQGIS 2.6 API is located at _apis/Python-2.6.api.

You can locate the documented class in the main C++ API and read about it. Then, look up the corresponding Python module and class using the PyQGIS API listing. In most cases, the C++ API name for a class is identical in Python.

Most of the time, it's easy to find the class that targets the functionality you need with most of QGIS being contained in the catch-all Core module. The more you use the API, the quicker you'll be able to locate the objects you need for your scripts.

Beware, however, that the results returned by this search engine may contain items you don't need and can even send you looking in the wrong direction because of the similar keywords in different modules.

The easiest approach to creating a plugin is to use the Plugin Builder plugin to jumpstart development. You can find it in the main QGIS plugin repository and install it.

A file browser dialog will appear; you can choose a folder in which you want to create your plugin. Select one of the folders called plugins within the python folder in either the main user directory or the QGIS program directory. The following examples are from a Windows machine. You should use the folder in your user directory, which is the preferred place for third-party plugins. QGIS standard plugins go in the main program directory:

This recipe shows you the bare bones needed to make a working plugin. Although we haven't altered it, the code for the plugin's behavior is contained in myplugin.py. You can change the icon and the GUI, and just recompile any time you want. Note that we must compile the Qt4 portion of the plugin, which creates the dialog box. The entire QGIS GUI is built on the Qt4 library, so the pyrrc4 compiler and pyuic4 is included to compile the GUI widgets.

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