Installing QGIS for development

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.

Getting ready

QGIS uses slightly different installation methods for Windows, GNU/Linux, and Mac OS X. The Windows installers install everything you need for Python development, including Python itself.

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 http://www.kyngchaos.com/software/python.

How to do it

On Linux, you have the option to compile from the source or you can just specify the Python QGIS interface to be installed through your package manager.

Installing PyQGIS using the Debian package manager

1. For Linux distributions based on the Debian Linux package manager, which includes Ubuntu and Debian, use the following command in a shell:

   sudo apt-get update
   

2. Next, install the QGIS, PyQGIS, and QGIS GRASS plugins:

   sudo apt-get install qgis python-qgis qgis-plugin-grass
   

Installing PyQGIS using the RPM package manager

1. For Linux distributions based on the Red Hat Package Manager (RPM), first update the package manager, as follows:

   sudo yum update
   

2. Then, install the packages for the QGIS, PyQGIS, and QGIS GRASS plugins:

   sudo yum install qgis qgis-python qgis-grass
   

Setting the environment variables

Now, we must set the PYTHONPATH to the PyQGIS directory. At the same time, append the path to this directory to the PATH variable so that you can use the PyQGIS modules with an external IDE.

Setting the environment variables on Windows

1. Set the PYTHONPATH variable in a command prompt to the bin directory of the QGIS installation:

   set PYTHONPATH="C:\Program Files\QGIS Brighton\bin"
   

2. Next, append QGIS's bin directories to the system's PATH variable:

   set PATH="C:\Program Files\QGIS Brighton\bin";"C:\Program Files\QGIS Brighton\bin\apps\qgis\bin";%PATH%
   

Setting the environment variables on Linux

1. Set the PYTHONPATH variable in a command prompt to the bin directory of the QGIS installation:

   export PYTHONPATH=/usr/share/qgis/python
   

2. Now, append the QGIS shared library directory to the runtime search path. Note that this location can vary depending on your particular system configuration:

   export LD_LIBRARY_PATH=/usr/share/qgis/python
   

How it works…

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.

There's more…

This recipe uses the default QGIS paths on each platform. If you aren't sure which PyQGIS path is for your system, you can figure this out from the Python console in QGIS.

Finding the PyQGIS path on Windows

The libraries on Windows are stored in a different location than in the case of other platforms. To locate the path, you can check the current working directory of the Python console:

1. Start QGIS.
2. Select Python Console from the Plugins menu, which appears in the lower-right corner of the QGIS application window, as shown in the following screenshot:
   
3. Use the os module to get the current working directory:

   import os
   os.getcwd()
   

4. Verify that the current working directory of the Python console is returned.

Finding the location of the QGIS Python installation on other platforms

Perform the following steps to find the path needed for this recipe on all the platforms besides Windows:

1. Start QGIS.
2. Start the QGIS Python Console.
3. Use the sys module to locate the PyQGIS path:

   import sys
   sys.path
   

4. Python will return a list of paths.
5. Find the path that ends in /python, which is the location of the Python installation used by QGIS