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Sviluppare Plugin Python

É possibile creare plugin nel linguaggio di programmazione Python. A differenza dei classici plugin scritti in C++ questi dovrebbero essere piú facili da scrivere, capire, mantenere e distribuire grazie alla natura dinamica del linguaggio Python.

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

  • UNIX/Mac: ~/.qgis2/python/plugins and (qgis_prefix)/share/qgis/python/plugins
  • Windows: ~/.qgis2/python/plugins and (qgis_prefix)/python/plugins

Home directory (denoted by above ~) on Windows is usually something like C:\Documents and Settings\(user) (on Windows XP or earlier) or C:\Users\(user). Since QGIS is using Python 2.7, subdirectories of these paths have to contain an __init__.py file to be considered Python packages that can be imported as plugins.

Nota

By setting QGIS_PLUGINPATH to an existing directory path, you can add this path to the list of paths that are searched for plugins.

Passi:

  1. Idea: Avere un’idea su cosa si vuole fare con il nuovo plugin QGIS. Perché lo fai? Esiste giá un altro plugin per questo problema?

  2. Create files: Create the files described next. A starting point (__init__.py). Fill in the Metadati del plugin (metadata.txt) A main python plugin body (mainplugin.py). A form in QT-Designer (form.ui), with its resources.qrc.
  3. Write code: Write the code inside the mainplugin.py
  4. Test: Chiudi e ri-apri QGIS e importa nuovamente il tuo plugin. Controlla se tutto va bene.

  5. Publish: Publish your plugin in QGIS repository or make your own repository as an “arsenal” of personal “GIS weapons”.

Scrivere un 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. The QGIS team also maintains an Repository di plugin python ufficiale. Ready to create a plugin but no idea what to do? Python Plugin Ideas wiki page lists wishes from the community!

File del plugin

Here’s the directory structure of our example plugin

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

Qual é il significato dei 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 = La traduzione in Python del file .qrc descritto sopra.

  • form.ui = The GUI created by Qt Designer.
  • form.py = La traduzione in Python del file form.ui descritto sopra.

  • metadata.txt = Required for QGIS >= 1.8.0. Contains general info, version, name and some other metadata used by plugins website and plugin infrastructure. Since QGIS 2.0 the metadata from __init__.py are not accepted anymore and the metadata.txt is required.

Here is an online automated way 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 Repository di plugin python ufficiale you must check that your plugin follows some additional rules, required for plugin Validazione

Contenuto del plugin

Qui puoi trovare informazioni ed esempi su cosa aggiungere in ognuno dei file nella struttura descritta sopra.

Metadati del plugin

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 to put this information.

Importante

Tutti i metadati devono codificati in UTF-8.

Nome del metadato

Obbligatorio

Note

nome

Vero

una string contenente il nome del plugin

qgisMinimumVersion

Vero

notazione puntata della versione minima di QGIS

qgisMaximumVersion

Falso

notazione puntata della massima versione di QGIS

descrizione

Vero

short text which describes the plugin, no HTML allowed
about

Vero

longer text which describes the plugin in details, no HTML allowed

versione

Vero

stringa con la notazione puntata della versione

autore

Vero

nome dell’autore

email

Vero

email of the author, not shown in the QGIS plugin manager or in the website unless by a registered logged in user, so only visible to other plugin authors and plugin website administrators

elenco cambiamenti

Falso

stringa, puó essere su piú righe, HTML non consentito

sperimentale

Falso

flag booleano, `True ` o `False `

dismesso

Falso

il flag booleano, True or False, si applica all’intero plugin e non solo alla versione caricata

etichette

Falso

comma separated list, spaces are allowed inside individual tags
homepage

Falso

una URL valida che punta alla homepage del plugin

repository

Vero

una URL valida per il repository del codice sorgente

tracker

Falso

una URL valida per i tickets ed i bug report

icona

Falso

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)

categoria

Falso

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

category=Raster

Nota

Se qgisMaximumVersion é vuoto, viene automaticamente impostato alla versione maggiore piú .99 quando viene caricato in Repository di plugin python ufficiale.

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 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=http://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=2.0

__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 to QGIS. It receives reference to instance of QgisInterface and must return instance 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

mainPlugin.py

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

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

# initialize Qt resources from file resources.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.setObjectName("testAction")
    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 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

You can see that in the above example, the 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:

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

Hanno tutti la stessa sintassi del metodo addPluginToMenu().

Aggiungere il menu del tuo plugin ad uno di questi metodi predefiniti é raccomandato per mantenere la consistenza riguardo all’organizzazione dei plugin. É comunque possibile aggiungere un gruppo personalizzato alla barra dei menu, come dimostrato nel prossimo esempio:

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")
    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()

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

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

Nota

In Windows environments, attempting to run the pyrcc4 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\pyrcc4.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 to your QGIS installation.

Documentazione

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.

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.

Software requirements

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

sudo apt-get install qt4-dev-tools

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.

.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. An example of the pro file is:

FORMS = ../ui/*

SOURCES = ../your_plugin.py

TRANSLATIONS = your_plugin_it.ts

In this particular case all your UIs are placed in the ../ui folder and you want to translate all of them.

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.

Avvertimento

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

.ts file

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

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

lupdate your_plugin.pro

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

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

.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).

Load the plugin

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

You should see your plugin in the correct language.

Avvertimento

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.