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.
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!
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:
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.
Here you can find information and examples about what to add in each of the files in the file structure described above.
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.
All metadata must be in UTF-8 encoding.
|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|
|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:
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@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
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:
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()
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.
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.