Qgis Syntax Plugins

qgis-syntax-plugins

Quick Reference

Plugin File Structure

Plugin Lifecycle

QGIS Startup
    |
    v
classFactory(iface) ---------> returns plugin instance
    |
    v
initGui() -------------------> create menus, toolbars, actions, dock widgets
    |
    v
[plugin active -- user interacts]
    |
    v
unload() --------------------> remove ALL GUI elements, disconnect ALL signals
    |
    v
Plugin deactivated

Plugin Installation Paths

Key iface Methods

Critical Warnings

ALWAYS implement unload() to remove ALL GUI elements, menu entries, toolbar icons, and dock widgets added in initGui(). Failure causes ghost UI elements that persist after plugin deactivation.

ALWAYS use self.iface.mainWindow() as the parent for QActions and dialogs. This ensures proper window management and garbage collection.

ALWAYS disconnect ALL signal connections in unload(). Dangling connections cause crashes when signals fire after plugin objects are destroyed.

NEVER access iface, QgsProject.instance(), or any GUI object from a background thread. This causes segfaults and silent crashes.

NEVER raise exceptions in QgsTask.run(). Return False to indicate failure instead.

NEVER leave compiled files (resources_rc.py, ui_*.py) in the repository. Generate them during build with pyrcc5 and pyuic5.

ALWAYS set objectName on QActions via action.setObjectName("uniqueName"). This prevents conflicts with other plugins and enables QGIS to save toolbar customizations.

ALWAYS track every GUI element you create in initGui() as an instance attribute so unload() can remove it.

Decision Tree: Plugin Type Selection

What does the plugin need to do?
|
+-- Add a toolbar button / menu item that runs a function?
|   --> Minimal plugin (QAction + run method)
|
+-- Show a dialog with input fields?
|   --> Dialog plugin (QAction + QDialog from .ui file)
|
+-- Show a persistent panel?
|   --> Dock widget plugin (QDockWidget added via iface.addDockWidget)
|
+-- Add a Processing algorithm?
|   --> Processing provider plugin (see qgis-syntax-processing-scripts)
|
+-- Interact with the map canvas?
|   --> Map tool plugin (QgsMapTool subclass)

Essential Patterns

Pattern 1: classFactory Entry Point

The __init__.py file MUST contain classFactory():

def classFactory(iface):
    """Load the plugin class. Called by QGIS on plugin startup."""
    from .mainPlugin import MyPlugin
    return MyPlugin(iface)

QGIS calls this function with the QgisInterface object. It MUST return an instance of the plugin class.

Pattern 2: Plugin Class Skeleton

from qgis.PyQt.QtWidgets import QAction
from qgis.PyQt.QtGui import QIcon
import os

class MyPlugin:
    def __init__(self, iface):
        self.iface = iface
        self.plugin_dir = os.path.dirname(__file__)
        self.actions = []

    def initGui(self):
        icon_path = os.path.join(self.plugin_dir, "icon.png")
        action = QAction(
            QIcon(icon_path),
            "My Plugin",
            self.iface.mainWindow()
        )
        action.setObjectName("myPluginAction")
        action.triggered.connect(self.run)
        self.iface.addToolBarIcon(action)
        self.iface.addPluginToMenu("&My Plugin", action)
        self.actions.append(action)

    def unload(self):
        for action in self.actions:
            self.iface.removePluginMenu("&My Plugin", action)
            self.iface.removeToolBarIcon(action)

    def run(self):
        pass

Pattern 3: metadata.txt Required Fields

[general]
name=My Plugin Name
qgisMinimumVersion=3.0
description=Short one-line description
about=Longer multi-line description
version=1.0.0
author=Author Name
email=author@example.com
repository=https://github.com/author/my-plugin

Required fields: name, qgisMinimumVersion, description, about, version, author, email, repository.

If qgisMaximumVersion is omitted, it defaults to major.99 (e.g., 3.99 for qgisMinimumVersion=3.0).

Pattern 4: Plugin Settings Storage

from qgis.core import QgsSettings

class MyPlugin:
    def __init__(self, iface):
        self.iface = iface
        self.settings = QgsSettings()

    def save_setting(self, key, value):
        self.settings.setValue(f"MyPlugin/{key}", value)

    def load_setting(self, key, default=None):
        return self.settings.value(f"MyPlugin/{key}", default)

ALWAYS prefix settings keys with the plugin name to avoid collisions.

Pattern 5: Qt Designer Dialog Integration

from qgis.PyQt import uic
import os

FORM_CLASS, _ = uic.loadUiType(
    os.path.join(os.path.dirname(__file__), "dialog.ui")
)

class MyDialog(QDialog, FORM_CLASS):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.setupUi(self)

Pattern 6: Resource Compilation

Define resources in resources.qrc:

<RCC>
  <qresource prefix="/plugins/myplugin">
    <file>icon.png</file>
  </qresource>
</RCC>

Compile: pyrcc5 -o resources.py resources.qrc

Import in plugin: from . import resources

Common Operations

Add to Specific Menu Category

# Vector menu
self.iface.addPluginToVectorMenu("&My Plugin", self.action)
self.iface.removePluginVectorMenu("&My Plugin", self.action)

# Raster menu
self.iface.addPluginToRasterMenu("&My Plugin", self.action)
self.iface.removePluginRasterMenu("&My Plugin", self.action)

# Database menu
self.iface.addPluginToDatabaseMenu("&My Plugin", self.action)
self.iface.removePluginDatabaseMenu("&My Plugin", self.action)

Add a Dock Widget

from qgis.PyQt.QtCore import Qt
from qgis.PyQt.QtWidgets import QDockWidget, QWidget

def initGui(self):
    self.dock = QDockWidget("My Panel", self.iface.mainWindow())
    self.dock.setObjectName("myPluginDock")
    self.dock.setWidget(QWidget())
    self.iface.addDockWidget(Qt.RightDockWidgetArea, self.dock)

def unload(self):
    self.iface.removeDockWidget(self.dock)
    del self.dock

Show Messages to Users

# Message bar (non-blocking)
from qgis.core import Qgis
self.iface.messageBar().pushMessage(
    "My Plugin", "Operation complete", level=Qgis.Success, duration=3
)

# Log messages (for debugging)
from qgis.core import QgsMessageLog
QgsMessageLog.logMessage("Debug info", "My Plugin", Qgis.Info)

Publishing Checklist

1. Plugin folder name: ASCII letters, digits, underscore, minus ONLY. NEVER start with a digit.
2. Package as ZIP: plugin.zip containing pluginfolder/ with all files.
3. Submit to https://plugins.qgis.org/ (requires OSGeo ID).
4. Staff approval required before publication.
5. Version MUST be unique across submissions.

Reference Links

• references/methods.md -- Plugin lifecycle methods, metadata.txt fields, QgisInterface methods
• references/examples.md -- Complete plugin skeletons, dialog plugins, dock widget plugins
• references/anti-patterns.md -- Plugin development pitfalls and fixes

Official Sources

• https://docs.qgis.org/latest/en/docs/pyqgis_developer_cookbook/plugins/index.html
• https://docs.qgis.org/latest/en/docs/pyqgis_developer_cookbook/plugins/plugins.html
• https://docs.qgis.org/latest/en/docs/pyqgis_developer_cookbook/plugins/releasing.html
• https://qgis.org/pyqgis/master/gui/QgisInterface.html