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Разработка расширений на Python

Для разработки расширений можно использовать язык программирования Python. По сравнению с классическими расширениями, написанными на C++, их легче разрабатывать, понимать, поддерживать и распространять в силу динамической природы самого Python.

Python plugins are listed together with C++ plugins in QGIS plugin manager. They are 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) (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.

Примечание

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

Шаги:

  1. Идея: Прежде всего нужна идея для нового расширения QGIS. Зачем это нужно? Какую задачу необходимо решить? Может, есть готовое расширения для решения этой задачи?

  2. Create files: Create the files described next. A starting point (__init__.py). Fill in the Plugin metadata (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. Тестирование: Закройте и снова откройте QGIS, загрузите своё расширение. Проверьте, что всё работает как надо.

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

Разработка расширения

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 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!

Plugin files

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*

Для чего используются файлы:

  • __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 = Required for QGIS >= 1.8.0. Containts 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.

Предупреждение

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

Plugin content

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

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 to put this information.

Важно

All metadata must be in UTF-8 encoding.

Metadata name Required Notes

имя

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

category=Raster

Примечание

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 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
; 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
tracker=http://bugs.itopen.it
repository=http://www.itopen.it/repo
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()

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")
    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.

Файл ресурсов

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.

При работе над реальным расширением удобно вести разработку в другом (рабочем) каталоге и создать makefile, который будет генерировать файлы интерфейса и ресурсов, а также выполнять копирование расширения в каталог QGIS.

Документация

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.

Кроме того, функция showPluginHelp() может принимать параметр packageName, идентифицирующий расширение, справку которого нужно отобразить; filename, который используется для переопределения имени файла с документацией; и section, для передачи имени якоря (закладки) в документе, на который браузер должен перейти.