Developing Python Plugins¶

Creating necessary 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. Contains general info, version, name and main class.
• 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 don’t require internet connection. Useful to help you start with a typical plugin.

__init__.py¶

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

def name():
  return "My testing plugin"

def description():
  return "This plugin has no real use."

def version():
  return "Version 0.1"

def qgisMinimumVersion():
  return "1.0"

def authorName():
  return "Developer"

def classFactory(iface):
  # load TestPlugin class from file testplugin.py
  from testplugin import TestPlugin
  return TestPlugin(iface)

In QGIS 1.9.90 plugins can be placed not only into Plugins menu but also into Raster, Vector, Database and Web menus. Therefore a new “category” metadata entry has been introduced. 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 __init__.py:

def category():
  return "Raster"

metadata.txt¶

For QGIS >= 1.8 you need to add a metadata.txt (see here) An exampe for this metadata.txt:

; the next section is mandatory
[general]
name=HelloWorld
qgisMinimumVersion=1.8
description=This is a plugin for greeting the
    (going multiline) world
category=Raster
version=version 1.2
; end of mandatory metadata

; start of optional metadata
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
; in a future version of the web application it will
; be probably possible to create a project on redmine
; if they are not filled
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

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!"

If you use QGIS 1.9.90 or higher and want to place your plugin into one of the new menus (Raster, Vector, Database or Web), you should modify the code of the initGui() and unload() functions. Since these new menus are available only in QGIS 1.9.90, our first step is to check if the running QGIS version has all necessary functions. If the new menus are available, we will place our plugin under this menu, otherwise we will use the old Plugins menu. Here is an example for Raster menu:

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)

  # check if Raster menu available
  if hasattr(self.iface, "addPluginToRasterMenu"):
    # Raster menu and toolbar available
    self.iface.addRasterToolBarIcon(self.action)
    self.iface.addPluginToRasterMenu("&Test plugins", self.action)
  else:
    # there is no Raster menu, place plugin under Plugins menu as usual
    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):
  # check if Raster menu available and remove our buttons from appropriate
  # menu and toolbar
  if hasattr(self.iface, "addPluginToRasterMenu"):
    self.iface.removePluginRasterMenu("&Test plugins",self.action)
    self.iface.removeRasterToolBarIcon(self.action)
  else:
    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)

A full list of methods that can be used to place plugin under these menus/toolbars is available in the API docs.

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

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 plugin manager and see a message in console when toolbar icon or appopriate 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.

How to call a method by a key shortcut¶

In the plug-in add to the initGui():

self.keyAction = QAction("Test Plugin", self.iface.mainWindow())
self.iface.registerMainWindowAction(self.keyAction, "F7") # action1 is triggered by the F7 key
self.iface.addPluginToMenu("&Test plugins", self.keyAction)
QObject.connect(self.keyAction, SIGNAL("triggered()"),self.keyActionF7)

To unload() add:

self.iface.unregisterMainWindowAction(self.keyAction)

The method that is called when F7 is pressed:

def keyActionF7(self):
  QMessageBox.information(self.iface.mainWindow(),"Ok", "You pressed F7")

How to toggle Layers (work around)¶

Note: from QGIS 1.5 there is QgsLegendInterface class that allows some manipulation with list of layers within legend.

As there is currently no method to directly access the layers in the legend, here is a workaround how to toggle the layers using layer transparency:

def toggleLayer(self, lyrNr):
  lyr = self.iface.mapCanvas().layer(lyrNr)
  if lyr:
    cTran = lyr.getTransparency()
    lyr.setTransparency(0 if cTran > 100 else 255)
    self.iface.mapCanvas().refresh()

The method requires the layer number (0 being the top most) and can be called by:

self.toggleLayer(3)

How to access attribute table of selected features¶

def changeValue(self, value):
  layer = self.iface.activeLayer()
  if(layer):
    nF = layer.selectedFeatureCount()
    if (nF > 0):
    layer.startEditing()
    ob = layer.selectedFeaturesIds()
    b = QVariant(value)
    if (nF > 1):
      for i in ob:
      layer.changeAttributeValue(int(i),1,b) # 1 being the second column
    else:
      layer.changeAttributeValue(int(ob[0]),1,b) # 1 being the second column
    layer.commitChanges()
    else:
      QMessageBox.critical(self.iface.mainWindow(),"Error", "Please select at least one feature from current layer")
  else:
    QMessageBox.critical(self.iface.mainWindow(),"Error","Please select a layer")

The method requires the one parameter (the new value for the attribute field of the selected feature(s)) and can be called by:

self.changeValue(50)

How to debug a plugin using PDB¶

First add this code in the spot where you would like to debug:

# Use pdb for debugging
import pdb
# These lines allow you to set a breakpoint in the app
pyqtRemoveInputHook()
pdb.set_trace()

Then run QGIS from the command line.

On Linux do:

$ ./Qgis

On Mac OS X do:

$ /Applications/Qgis.app/Contents/MacOS/Qgis

And when the application hits your breakpoint you can type in the console!