.. _introduction: .. highlight:: python :linenothreshold: 5 ************ Introduction ************ This document is intended to be both a tutorial and a reference guide. While it does not list all possible use cases, it should give a good overview of the principal functionality. .. contents:: :local: Python support was first introduced in QGIS 0.9. There are several ways to use Python in QGIS Desktop (covered in the following sections): * Issue commands in the Python console within QGIS * Create and use plugins * Automatically run Python code when QGIS starts * Create processing algorithms * Create functions for expressions in QGIS * Create custom applications based on the QGIS API Python bindings are also available for QGIS Server, including Python plugins (see :ref:`server_plugins`) and Python bindings that can be used to embed QGIS Server into a Python application. .. index:: API There is a :api:`complete QGIS API <>` reference that documents the classes from the QGIS libraries. :pyqgis:`The Pythonic QGIS API (pyqgis) <>` is nearly identical to the C++ API. A good resource for learning how to perform common tasks is to download existing plugins from the `plugin repository `_ and examine their code. .. index:: pair: Python; Console .. _pythonconsole: Scripting in the Python Console =============================== QGIS provides an integrated :ref:`Python console ` for scripting. It can be opened from the :menuselection:`Plugins --> Python Console` menu: .. figure:: img/console.png :align: center :width: 40em QGIS Python console The screenshot above illustrates how to get the layer currently selected in the layer list, show its ID and optionally, if it is a vector layer, show the feature count. For interaction with the QGIS environment, there is an :data:`iface` variable, which is an instance of :class:`QgisInterface `. This interface allows access to the map canvas, menus, toolbars and other parts of the QGIS application. For user convenience, the following statements are executed when the console is started (in the future it will be possible to set further initial commands) :: from qgis.core import * import qgis.utils For those which use the console often, it may be useful to set a shortcut for triggering the console (within :menuselection:`Settings --> Keyboard shortcuts...`) .. index:: Python; Plugins Python Plugins ============== The functionality of QGIS can be extended using plugins. Plugins can be written in Python. The main advantage over C++ plugins is simplicity of distribution (no compiling for each platform) and easier development. Many plugins covering various functionality have been written since the introduction of Python support. The plugin installer allows users to easily fetch, upgrade and remove Python plugins. See the `Python Plugins `_ page for more information about plugins and plugin development. Creating plugins in Python is simple, see :ref:`developing_plugins` for detailed instructions. .. note:: Python plugins are also available for QGIS server. See :ref:`server_plugins` for further details. .. index:: pair: Python; Startup Running Python code when QGIS starts ==================================== There are two distinct methods to run Python code every time QGIS starts. 1. Creating a startup.py script 2. Setting the ``PYQGIS_STARTUP`` environment variable to an existing Python file The :file:`startup.py` file ---------------------------- Every time QGIS starts, the user's Python home directory * Linux: :file:`.local/share/QGIS/QGIS3` * Windows: :file:`AppData\\Roaming\\QGIS\\QGIS3` * macOS: :file:`Library/Application Support/QGIS/QGIS3` is searched for a file named :file:`startup.py`. If that file exists, it is executed by the embedded Python interpreter. .. note:: The default path depends on the operating system. To find the path that will work for you, open the Python Console and run ``QStandardPaths.standardLocations(QStandardPaths.AppDataLocation)`` to see the list of default directories. .. index:: pair: Environment; PYQGIS_STARTUP The PYQGIS_STARTUP environment variable --------------------------------------- You can run Python code just before QGIS initialization completes by setting the ``PYQGIS_STARTUP`` environment variable to the path of an existing Python file. This code will run before QGIS initialization is complete. This method is very useful for cleaning sys.path, which may have undesireable paths, or for isolating/loading the initial environment without requiring a virtual environment, e.g. homebrew or MacPorts installs on Mac. .. index:: pair: Python; Custom applications pair: Python; Standalone scripts .. _pythonapplications: Python Applications =================== It is often handy to create scripts for automating processes. With PyQGIS, this is perfectly possible --- import the :mod:`qgis.core` module, initialize it and you are ready for the processing. Or you may want to create an interactive application that uses GIS functionality --- perform measurements, export a map as PDF, ... The :mod:`qgis.gui` module provides various GUI components, most notably the map canvas widget that can be incorporated into the application with support for zooming, panning and/or any further custom map tools. PyQGIS custom applications or standalone scripts must be configured to locate the QGIS resources, such as projection information and providers for reading vector and raster layers. QGIS Resources are initialized by adding a few lines to the beginning of your application or script. The code to initialize QGIS for custom applications and standalone scripts is similar. Examples of each are provided below. .. note:: Do *not* use :file:`qgis.py` as a name for your script. Python will not be able to import the bindings as the script's name will shadow them. .. _standalonescript: Using PyQGIS in standalone scripts ---------------------------------- To start a standalone script, initialize the QGIS resources at the beginning of the script: .. code-block:: python from qgis.core import * # Supply path to qgis install location QgsApplication.setPrefixPath("/path/to/qgis/installation", True) # Create a reference to the QgsApplication. Setting the # second argument to False disables the GUI. qgs = QgsApplication([], False) # Load providers qgs.initQgis() # Write your code here to load some layers, use processing # algorithms, etc. # Finally, exitQgis() is called to remove the # provider and layer registries from memory qgs.exitQgis() First we import the :mod:`qgis.core` module and configure the prefix path. The prefix path is the location where QGIS is installed on your system. It is configured in the script by calling the :meth:`setPrefixPath ` method. The second argument of :meth:`setPrefixPath ` is set to ``True``, specifying that default paths are to be used. The QGIS install path varies by platform; the easiest way to find it for your system is to use the :ref:`pythonconsole` from within QGIS and look at the output from running ``QgsApplication.prefixPath()``. After the prefix path is configured, we save a reference to ``QgsApplication`` in the variable ``qgs``. The second argument is set to ``False``, specifying that we do not plan to use the GUI since we are writing a standalone script. With ``QgsApplication`` configured, we load the QGIS data providers and layer registry by calling the ``qgs.initQgis()`` method. With QGIS initialized, we are ready to write the rest of the script. Finally, we wrap up by calling ``qgs.exitQgis()`` to remove the data providers and layer registry from memory. Using PyQGIS in custom applications ----------------------------------- The only difference between :ref:`standalonescript` and a custom PyQGIS application is the second argument when instantiating the ``QgsApplication``. Pass ``True`` instead of ``False`` to indicate that we plan to use a GUI. .. code-block:: python from qgis.core import * # Supply the path to the qgis install location QgsApplication.setPrefixPath("/path/to/qgis/installation", True) # Create a reference to the QgsApplication. # Setting the second argument to True enables the GUI. We need # this since this is a custom application. qgs = QgsApplication([], True) # load providers qgs.initQgis() # Write your code here to load some layers, use processing # algorithms, etc. # Finally, exitQgis() is called to remove the # provider and layer registries from memory qgs.exitQgis() Now you can work with the QGIS API - load layers and do some processing or fire up a GUI with a map canvas. The possibilities are endless :-) .. index:: pair: Custom applications; Running Running Custom Applications --------------------------- You need to tell your system where to search for QGIS libraries and appropriate Python modules if they are not in a well-known location - otherwise Python will complain:: >>> import qgis.core ImportError: No module named qgis.core This can be fixed by setting the ``PYTHONPATH`` environment variable. In the following commands, ```` should be replaced with your actual QGIS installation path: * on Linux: :command:`export PYTHONPATH=//share/qgis/python` * on Windows: :command:`set PYTHONPATH=c:\\\\python` * on macOS: :command:`export PYTHONPATH=//Contents/Resources/python` Now, the path to the PyQGIS modules is known, but they depend on the ``qgis_core`` and ``qgis_gui`` libraries (the Python modules serve only as wrappers). The path to these libraries may be unknown to the operating system, and then you will get an import error again (the message might vary depending on the system):: >>> import qgis.core ImportError: libqgis_core.so.3.2.0: cannot open shared object file: No such file or directory Fix this by adding the directories where the QGIS libraries reside to the search path of the dynamic linker: * on Linux: :command:`export LD_LIBRARY_PATH=//lib` * on Windows: :command:`set PATH=C:\\\\bin;C:\\\\apps\\\\bin;%PATH%` where ```` should be replaced with the type of release you are targeting (eg, ``qgis-ltr``, ``qgis``, ``qgis-dev``) These commands can be put into a bootstrap script that will take care of the startup. When deploying custom applications using PyQGIS, there are usually two possibilities: * require the user to install QGIS prior to installing your application. The application installer should look for default locations of QGIS libraries and allow the user to set the path if not found. This approach has the advantage of being simpler, however it requires the user to do more steps. * package QGIS together with your application. Releasing the application may be more challenging and the package will be larger, but the user will be saved from the burden of downloading and installing additional pieces of software. The two deployment models can be mixed. You can provide a standalone applications on Windows and macOS, but for Linux leave the installation of GIS up to the user and his package manager. Technical notes on PyQt and SIP =============================== We've decided for Python as it's one of the most favoured languages for scripting. PyQGIS bindings in QGIS 3 depend on SIP and PyQt5. The reason for using SIP instead of the more widely used SWIG is that the QGIS code depends on Qt libraries. Python bindings for Qt (PyQt) are done using SIP and this allows seamless integration of PyQGIS with PyQt.