1. 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.
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 QGIS Server and Python) and Python bindings that can be used to embed QGIS Server into a Python application.
There is a complete QGIS API reference that documents the classes from the QGIS libraries. 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.
1.1. Scripting in the Python Console¶
QGIS provides an integrated Python console for scripting. It can be opened from the menu:
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 iface
variable, which is an instance of
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
)1.2. 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 Developing Python Plugins for detailed instructions.
Note
Python plugins are also available for QGIS server. See QGIS Server and Python for further details.
1.3. Running Python code when QGIS starts¶
There are two distinct methods to run Python code every time QGIS starts.
Creating a startup.py script
Setting the
PYQGIS_STARTUP
environment variable to an existing Python file
1.3.1. The startup.py
file¶
Every time QGIS starts, the user’s Python home directory
Linux:
.local/share/QGIS/QGIS3
Windows:
AppData\Roaming\QGIS\QGIS3
macOS:
Library/Application Support/QGIS/QGIS3
is searched for a file named 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.
1.3.2. 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.
1.4. Python Applications¶
It is often handy to create scripts for automating processes.
With PyQGIS, this is perfectly possible — import
the 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 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 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.
1.4.1. Using PyQGIS in standalone scripts¶
To start a standalone script, initialize the QGIS resources at the beginning of the script:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | 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 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 setPrefixPath
method.
The second argument of
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 Scripting in the Python Console 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.
1.4.2. Using PyQGIS in custom applications¶
The only difference between Using PyQGIS in standalone scripts 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.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | 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 :-)
1.4.3. 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, <qgispath>
should be replaced with your actual
QGIS installation path:
on Linux: export PYTHONPATH=/<qgispath>/share/qgis/python
on Windows: set PYTHONPATH=c:\<qgispath>\python
on macOS: export PYTHONPATH=/<qgispath>/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: export LD_LIBRARY_PATH=/<qgispath>/lib
on Windows: set PATH=C:\<qgispath>\bin;C:\<qgispath>\apps\<qgisrelease>\bin;%PATH% where
<qgisrelease>
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.
1.5. 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.