16.1. Structuring Python Plugins

In order to create a plugin, here are some steps to follow:

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?
Create files: some are essentials (see Liitännäisen tiedostot)
Write code: Write the code in appropriate files
Test: Reload your plugin to check if everything is OK
Publish: Publish your plugin in QGIS repository or make your own repository as an ”arsenal” of personal ”GIS weapons”.

16.1.1. Writing a plugin 

Since the introduction of Python plugins in QGIS, a number of plugins have appeared. The QGIS team maintains an Official Python plugin repository. You can use their source to learn more about programming with PyQGIS or find out whether you are duplicating development effort.

16.1.1.1. Liitännäisen tiedostot 

Here’s the directory structure of our example plugin

PYTHON_PLUGINS_PATH/
  MyPlugin/
    __init__.py    --> *required*
    mainPlugin.py  --> *core code*
    metadata.txt   --> *required*
    resources.qrc  --> *likely useful*
    resources.py   --> *compiled version, likely useful*
    form.ui        --> *likely useful*
    form.py        --> *compiled version, likely useful*

What is the meaning of the files:

__init__.py = The starting point of the plugin. It has to have the classFactory() method and may have any other initialisation code.
mainPlugin.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 = Contains general info, version, name and some other metadata used by plugins website and plugin infrastructure.

Here is a way of creating the basic files (skeleton) of a typical QGIS Python plugin.

There is a QGIS plugin called Plugin Builder 3 that creates a plugin template for QGIS. This is the recommended option, as it produces 3.x compatible sources.

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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

16.1.2. Plugin content 

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

16.1.2.1. Plugin metadata 

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

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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	short text which describes the plugin, no HTML allowed
about	True	longer text which describes the plugin in details, no HTML allowed
version	True	short string with the version dotted notation
author	True	author name
email	True	email of the author, only shown on the website to logged in users, but visible in the Plugin Manager after the plugin is installed
changelog	False	string, can be multiline, no HTML allowed
experimental	False	boolean flag, `True` or `False` - `True` if this version is experimental
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 allowed inside individual tags
homepage	False	a valid URL pointing to the homepage of your plugin
repository	True	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) of a web friendly image (PNG, JPEG)
category	False	one of `Raster`, `Vector`, `Database` and `Web`
plugin_dependencies	False	PIP-like comma separated list of other plugins to install
server	False	boolean flag, `True` or `False`, determines if the plugin has a server interface
hasProcessingProvider	False	boolean flag, `True` or `False`, determines if the plugin provides processing algorithms

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 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 or Web. For example, if your plugin will be available from Raster menu, add this to metadata.txt

category=Raster

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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=me@example.com
author=Just Me
qgisMinimumVersion=3.0
description=This is an example plugin for greeting the world.
    Multiline is allowed:
    lines starting with spaces belong to the same
    field, in this case to the "description" field.
    HTML formatting is not allowed.
about=This paragraph can contain a detailed description
    of the plugin. Multiline is allowed, HTML is not.
version=version 1.2
tracker=http://bugs.itopen.it
repository=http://www.itopen.it/repo
; end of mandatory metadata

; start of optional metadata
category=Raster
changelog=The changelog lists the plugin versions
    and their changes as in the example below:
    1.0 - First stable release
    0.9 - All features implemented
    0.8 - First testing release

; Tags are in comma separated value format, spaces are allowed within the
; tag name.
; Tags should be in English language. Please also check for existing tags and
; synonyms before creating a new one.
tags=wkt,raster,hello world

; these metadata can be empty, they will eventually become mandatory.
homepage=https://www.itopen.it
icon=icon.png

; experimental flag (applies to the single version)
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=3.99

; Since QGIS 3.8, a comma separated list of plugins to be installed
; (or upgraded) can be specified.
; The example below will try to install (or upgrade) "MyOtherPlugin" version 1.12
; and any version of "YetAnotherPlugin"
plugin_dependencies=MyOtherPlugin==1.12,YetAnotherPlugin

16.1.2.2. init.py 

This file is required by Python’s import system. Also, QGIS requires that this file contains a classFactory() function, which is called when the plugin gets loaded into QGIS. It receives a reference to the instance of QgisInterface and must return an object of your plugin’s class from the mainplugin.py — in our case it’s called TestPlugin (see below). This is how __init__.py should look like

def classFactory(iface):
  from .mainPlugin import TestPlugin
  return TestPlugin(iface)

# any other initialisation needed

16.1.2.3. mainPlugin.py 

This is where the magic happens and this is how magic looks like: (e.g. mainPlugin.py)

from qgis.PyQt.QtGui import *
from qgis.PyQt.QtWidgets import *

# initialize Qt resources from file resources.py
from . 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.setObjectName("testAction")
    self.action.setWhatsThis("Configuration for test plugin")
    self.action.setStatusTip("This is status tip")
    self.action.triggered.connect(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
    self.iface.mapCanvas().renderComplete.connect(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
    self.iface.mapCanvas().renderComplete.disconnect(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 in the main plugin source file (e.g. mainPlugin.py) are:

__init__ which gives access to QGIS interface
initGui() called when the plugin is loaded
unload() called when the plugin is unloaded

In the above example, addPluginToMenu() 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.setObjectName("testMenu")
    self.menu.setTitle("MyMenu")

    self.action = QAction(QIcon(":/plugins/testplug/icon.png"),
                          "Test plugin",
                          self.iface.mainWindow())
    self.action.setObjectName("testAction")
    self.action.setWhatsThis("Configuration for test plugin")
    self.action.setStatusTip("This is status tip")
    self.action.triggered.connect(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()

Don’t forget to set QAction and QMenu objectName to a name specific to your plugin so that it can be customized.

16.1.2.4. 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 pyrcc5 command:

pyrcc5 -o resources.py resources.qrc

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In Windows environments, attempting to run the pyrcc5 from Command Prompt or Powershell will probably result in the error ”Windows cannot access the specified device, path, or file […]”. The easiest solution is probably to use the OSGeo4W Shell but if you are comfortable modifying the PATH environment variable or specifiying the path to the executable explicitly you should be able to find it at <Your QGIS Install Directory>\bin\pyrcc5.exe.

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 into your QGIS installation.

16.1.3. 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.

16.1.4. Translation 

With a few steps you can set up the environment for the plugin localization so that depending on the locale settings of your computer the plugin will be loaded in different languages.

16.1.4.1. Software requirements 

The easiest way to create and manage all the translation files is to install Qt Linguist. In a Debian-based GNU/Linux environment you can install it typing:

sudo apt install qttools5-dev-tools

16.1.4.2. Files and directory 

When you create the plugin you will find the i18n folder within the main plugin directory.

All the translation files have to be within this directory.

16.1.4.2.1. .pro file 

First you should create a .pro file, that is a project file that can be managed by Qt Linguist.

In this .pro file you have to specify all the files and forms you want to translate. This file is used to set up the localization files and variables. A possible project file, matching the structure of our example plugin:

FORMS = ../form.ui
SOURCES = ../your_plugin.py
TRANSLATIONS = your_plugin_it.ts

Your plugin might follow a more complex structure, and it might be distributed across several files. If this is the case, keep in mind that pylupdate5, the program we use to read the .pro file and update the translatable string, does not expand wild card characters, so you need to place every file explicitly in the .pro file. Your project file might then look like something like this:

FORMS = ../ui/about.ui ../ui/feedback.ui \
        ../ui/main_dialog.ui
SOURCES = ../your_plugin.py ../computation.py \
          ../utils.py

Furthermore, the your_plugin.py file is the file that calls all the menu and sub-menus of your plugin in the QGIS toolbar and you want to translate them all.

Finally with the TRANSLATIONS variable you can specify the translation languages you want.

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Be sure to name the ts file like your_plugin_ + language + .ts otherwise the language loading will fail! Use the 2 letter shortcut for the language (it for Italian, de for German, etc…)

16.1.4.2.2. .ts file 

Once you have created the .pro you are ready to generate the .ts file(s) for the language(s) of your plugin.

Open a terminal, go to your_plugin/i18n directory and type:

pylupdate5 your_plugin.pro

you should see the your_plugin_language.ts file(s).

Open the .ts file with Qt Linguist and start to translate.

16.1.4.2.3. .qm file 

When you finish to translate your plugin (if some strings are not completed the source language for those strings will be used) you have to create the .qm file (the compiled .ts file that will be used by QGIS).

Just open a terminal cd in your_plugin/i18n directory and type:

lrelease your_plugin.ts

now, in the i18n directory you will see the your_plugin.qm file(s).

16.1.4.3. Translate using Makefile 

Alternatively you can use the makefile to extract messages from python code and Qt dialogs, if you created your plugin with Plugin Builder. At the beginning of the Makefile there is a LOCALES variable:

LOCALES = en

Add the abbreviation of the language to this variable, for example for Hungarian language:

LOCALES = en hu

Now you can generate or update the hu.ts file (and the en.ts too) from the sources by:

make transup

After this, you have updated .ts file for all languages set in the LOCALES variable. Use Qt Linguist to translate the program messages. Finishing the translation the .qm files can be created by the transcompile:

make transcompile

You have to distribute .ts files with your plugin.

16.1.4.4. Load the plugin 

In order to see the translation of your plugin, open QGIS, change the language (Settings ► Options ► General) and restart QGIS.

You should see your plugin in the correct language.

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If you change something in your plugin (new UIs, new menu, etc..) you have to generate again the update version of both .ts and .qm file, so run again the command of above.

my_plugin = qgis.utils.plugins['My Plugin']

16.1.5.3. Log Messages 

Plugins have their own tab within the Log Messages Panel.

16.1. Structuring Python Plugins

16.1.1. Writing a plugin 

16.1.1.1. Liitännäisen tiedostot 

16.1.2. Plugin content 

16.1.2.1. Plugin metadata 

16.1.2.2. init.py 

16.1.2.3. mainPlugin.py 

16.1.2.4. Resource File 

16.1.3. Documentation 

16.1.4. Translation 

16.1.4.1. Software requirements 

16.1.4.2. Files and directory 

16.1.4.2.1. .pro file 

16.1.4.2.2. .ts file 

16.1.4.2.3. .qm file 

16.1.4.3. Translate using Makefile 

16.1.4.4. Load the plugin 

16.1.5. Tips and Tricks 

16.1.5.1. Plugin Reloader 

16.1.5.2. Accessing Plugins 

16.1.5.3. Log Messages 