Developing Python Plugins

It is possible to create plugins in Python programming language. In comparison with classical plugins written in C++ these should be easier to write, understand, maintain and distribute due the dynamic nature of the Python language.

Python plugins are listed together with C++ plugins in QGIS plugin manager. They’re being searched for in these paths:

  • UNIX/Mac: ~/.qgis/python/plugins and (qgis_prefix)/share/qgis/python/plugins
  • Windows: ~/.qgis/python/plugins and (qgis_prefix)/python/plugins

Home directory (denoted by above ~) on Windows is usually something like C:\Documents and Settings\(user). Subdirectories of these paths are considered as Python packages that can be imported to QGIS as plugins.

Steps:

  1. Idea: Have an idea about what you want to do with your new QGIS plugin. Why do you do it? What problem do you want to solve? Is there already another plugin for that problem?
  2. Create files: Create the files described next. A starting point (__init.py__). Fill in the Plugin metadata (metadata.txt) A main python plugin body (plugin.py). A form in QT-Designer (form.ui), with its resources.qrc.
  3. Write code: Write the code inside the plugin.py
  4. Test: Close and re-open QGIS and import your plugin again. Check if everything is OK.
  5. Publish: Publish your plugin in QGIS repository or make your own repository as an “arsenal” of personal “GIS weapons”

Writing a plugin

Since the introduction of python plugins in QGIS, a number of plugins have appeared - on Plugin Repositories wiki page you can find some of them, you can use their source to learn more about programming with PyQGIS or find out whether you are not duplicating development effort. QGIS team also maintains an Official python plugin repository. Ready to create a plugin but no idea what to do? Python Plugin Ideas wiki page lists wishes from the community!

Plugin files

Here’s the directory structure of our example plugin:

PYTHON_PLUGINS_PATH/
  testplug/
    __init__.py
    plugin.py
    metadata.txt
    resources.qrc
    resources.py
    form.ui
    form.py

What is the meaning of the files:

  • __init__.py = The starting point of the plugin. It is normally empty.
  • plugin.py = The main working code of the plugin. Contains all the information about the actions of the plugin and the main code.
  • resources.qrc = The .xml document created by QT-Designer. Contains relative paths to resources of the forms.
  • resources.py = The translation of the .qrc file described above to Python.
  • form.ui = The GUI created by QT-Designer.
  • form.py = The translation of the form.ui described above to Python.
  • metadata.txt = Required for QGIS >= 1.8.0. Containts general info, version, name and some other metadata used by plugins website and plugin infrastructure. Metadata in metadata.txt is preferred to the methods in __init__.py. If the text file is present, it is used to fetch the values. From QGIS 2.0 the metadata from __init__.py will not be accepted and the metadata.txt file will be required.

Here and there are two automated ways of creating the basic files (skeleton) of a typical QGIS Python plugin.

Also there is a QGIS plugin called Plugin Builder that creates plugin template from QGIS and doesn’t require internet connection. This is the recommended option, as it produces 2.0 compatible sources.

Avvertimento

If you plan to upload the plugin to the Official python plugin repository you must check that your plugin follows some additional rules, required for plugin Validation

Plugin content

Here you can find information and examples about what to add in each of the files in the file structure described above.

Plugin metadata

First, plugin manager needs to retrieve some basic information about the plugin such as its name, description etc. File metadata.txt is the right place where to put this information.

Importante

All metadata must be in UTF-8 encoding.

Metadata name Required Notes
name True a short string containing the name of the plugin
qgisMinimumVersion True dotted notation of minimum QGIS version
qgisMaximumVersion False dotted notation of maximum QGIS version
description True longer text which describes the plugin, no HTML allowed
version True short string with the version dotted notation
author True author name
email True email of the author, will not be shown on the web site
changelog False string, can be multiline, no HTML allowed
experimental False boolean flag, True or False
deprecated False boolean flag, True or False, applies to the whole plugin and not just to the uploaded version
tags False comma separated list, spaces are allowe inside individual tags
homepage False a valid URL pointing to the homepage of your plugin
repository False a valid URL for the source code repository
tracker False a valid URL for tickets and bug reports
icon False a file name or a relative path (relative to the base folder of the plugin’s compressed package)
category False one of Raster, Vector, Database and Web

By default, plugins are placed in the Plugins menu (we will see in the next section how to add a menu entry for your plugin) but they can also be placed the into Raster, Vector, Database and Web menus. A corresponding “category” metadata entry exists to specify that, so the plugin can be classified accordingly. This metadata entry is used as tip for users and tells them where (in which menu) the plugin can be found. Allowed values for “category” are: Vector, Raster, Database, Web and Layers. For example, if your plugin will be available from Raster menu, add this to metadata.txt:

category=Raster

Nota

If qgisMaximumVersion is empty, it will be automatically set to the major version plus .99 when uploaded to the Official python plugin repository.

An example for this metadata.txt:

; the next section is mandatory

[general]
name=HelloWorld
[email protected]
author=Just Me
qgisMinimumVersion=2.0
description=This is a plugin for greeting the
    (going multiline) world
version=version 1.2
; end of mandatory metadata

; start of optional metadata
category=Raster
changelog=this is a very
    very
    very
    very
    very
    very long multiline changelog

; tags are in comma separated value format, spaces are allowed
tags=wkt,raster,hello world

; these metadata can be empty
homepage=http://www.itopen.it
tracker=http://bugs.itopen.it
repository=http://www.itopen.it/repo
icon=icon.png

; experimental flag
experimental=True

; deprecated flag (applies to the whole plugin and not only to the uploaded version)
deprecated=False

; if empty, it will be automatically set to major version + .99
qgisMaximumVersion=2.0

plugin.py

One thing worth mentioning is classFactory() function which is called when the plugin gets loaded to QGIS. It receives reference to instance of QgisInterface and must return instance of your plugin - in our case it’s called TestPlugin. This is how should this class look like (e.g. testplugin.py):

from PyQt4.QtCore import *
from PyQt4.QtGui import *
from qgis.core import *

# initialize Qt resources from file resouces.py
import resources

class TestPlugin:

  def __init__(self, iface):
    # save reference to the QGIS interface
    self.iface = iface

  def initGui(self):
    # create action that will start plugin configuration
    self.action = QAction(QIcon(":/plugins/testplug/icon.png"), "Test plugin", \
      self.iface.mainWindow())
    self.action.setWhatsThis("Configuration for test plugin")
    self.action.setStatusTip("This is status tip")
    QObject.connect(self.action, SIGNAL("triggered()"), self.run)

    # add toolbar button and menu item
    self.iface.addToolBarIcon(self.action)
    self.iface.addPluginToMenu("&Test plugins", self.action)

    # connect to signal renderComplete which is emitted when canvas
    # rendering is done
    QObject.connect(self.iface.mapCanvas(), SIGNAL("renderComplete(QPainter *)"), \
      self.renderTest)

  def unload(self):
    # remove the plugin menu item and icon
    self.iface.removePluginMenu("&Test plugins",self.action)
    self.iface.removeToolBarIcon(self.action)

    # disconnect form signal of the canvas
    QObject.disconnect(self.iface.mapCanvas(), SIGNAL("renderComplete(QPainter *)"), \
      self.renderTest)

  def run(self):
    # create and show a configuration dialog or something similar
    print "TestPlugin: run called!"

  def renderTest(self, painter):
    # use painter for drawing to map canvas
    print "TestPlugin: renderTest called!"

The only plugin functions that must exist are initGui() and unload(). These functions are called when the plugin is loaded and unloaded.

You can see that in the above example, the addPluginMenu() is used. This will add the corresponding menu action to the Plugins menu. Alternative methods exist to add the action to a different menu. Here is a list of those methods:

  • addPluginToRasterMenu()
  • addPluginToVectorMenu()
  • addPluginToDatabaseMenu()
  • addPluginToWebMenu()

All of them have the same syntax as the addPluginToMenu() method.

Adding your plugin menu to one of those predefined method is recommended to keep consistency in how plugin entries are organized. However, you can add your custom menu group directly to the menu bar, as the next example demonstrates:

def initGui(self):
    self.menu = QMenu(self.iface.mainWindow())
    self.menu.setTitle("MyMenu")

    self.action = QAction(QIcon(":/plugins/testplug/icon.png"), "Test plugin", \
      self.iface.mainWindow())
    self.action.setWhatsThis("Configuration for test plugin")
    self.action.setStatusTip("This is status tip")
    QObject.connect(self.action, SIGNAL("triggered()"), self.run)
    self.menu.addAction(self.action)

    menuBar = self.iface.mainWindow().menuBar()
    menuBar.insertMenu(self.iface.firstRightStandardMenu().menuAction(), self.menu)

def unload(self):
    self.menu.deleteLater()

Resource File

You can see that in initGui() we’ve used an icon from the resource file (called resources.qrc in our case):

<RCC>
  <qresource prefix="/plugins/testplug" >
     <file>icon.png</file>
  </qresource>
</RCC>

It is good to use a prefix that will not collide with other plugins or any parts of QGIS, otherwise you might get resources you did not want. Now you just need to generate a Python file that will contain the resources. It’s done with pyrcc4 command:

pyrcc4 -o resources.py resources.qrc

And that’s all... nothing complicated :) If you’ve done everything correctly you should be able to find and load your plugin in the plugin manager and see a message in console when toolbar icon or appropriate menu item is selected.

When working on a real plugin it’s wise to write the plugin in another (working) directory and create a makefile which will generate UI + resource files and install the plugin to your QGIS installation.

Documentation

The documentation for the plugin can be written as HTML help files. The qgis.utils module provides a function, showPluginHelp() which will open the help file browser, in the same way as other QGIS help.

The showPluginHelp`() function looks for help files in the same directory as the calling module. It will look for, in turn, index-ll_cc.html, index-ll.html, index-en.html, index-en_us.html and index.html, displaying whichever it finds first. Here ll_cc is the QGIS locale. This allows multiple translations of the documentation to be included with the plugin.

The showPluginHelp() function can also take parameters packageName, which identifies a specific plugin for which the help will be displayed, filename, which can replace “index” in the names of files being searched, and section, which is the name of an html anchor tag in the document on which the browser will be positioned.