Developing Python Plugins¶

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.

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.

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

An example for this metadata.txt:

; the next section is mandatory

[general]
name=HelloWorld
email=me@example.com
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.