Below is a list of fixes to common PyQGIS migration issues I collated whilst migrating a QGIS 2.18 plugin to QGIS 3.2 (and then to 3.4). I documented each error message and fix to make things easier if I came across the same issue again, and subsequently figured it may be of user to others who were migrating their plugins too.

• Error messages and fixes
  • Attribute errors
  • Name errors
  • Type errors
• See also (other hints/tips for plugin migration and associated troubleshooting)
• Contributions
• License

Details of error messages are documented as sub-headings, with fixes and examples for each listed underneath. Where possible I've included links back to relevant documentation too.

Error messages are listed alphabetically, and as such are organised by error type (AttributeError, NameError, TypeError).

To fix: replace Qgis.<Old Type> with QgsWkbTypes.<New Type>

e.g.

QGis.Line

# becomes
QgsWkbTypes.LineGeometry


Qgis.WKBPolygon 

# becomes
QgsWkbTypes.Polygon


Qgis.WKBPoint

# becomes
QgsWkbTypes.Point

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsWkbTypes

and QGIS breaking changes 'General changes' section

To fix: replace .fromMultiPolyline([qgs_point_list]) with .fromMultiPolylineXY([qgs_pointXY_list])

e.g.

QgsGeometry.fromMultiPolyline(qgs_point_list)

# becomes
QgsGeometry.fromMultiPolylineXY(qgs_point_xy_list)

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsGeometry.fromMultiPolylineXY()

and QGIS breaking changes for QgsGeometry

To fix: replace QgsMessageBar.<MessageLevel> with Qgis.<MessageLevel>

e.g.

QgsMessageBar.INFO

# becomes
Qgis.Info


QgsMessageBar.WARNING

# becomes
Qgis.Warning


QgsMessageBar.CRITICAL

# becomes
Qgis.Critical


QgsMessageBar.SUCCESS

# becomes
Qgis.Success

See also: QGIS API doc for Qgis.MessageLevel

and QGIS breaking changes for QgsMessageLevel

To fix: replace QgsSymbolLayerRegistry.instance() with QgsApplication.symbolLayerRegistry()

e.g.

QgsSymbolLayerRegistry.instance()

# becomes
QgsApplication.symbolLayerRegistry()

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsApplication.symbolLayerRegistry()

and QGIS breaking changes for QgsSymbolLayerRegistry

To fix: reference the layer (by id) within the layerTreeRoot, and set visibility there.

e.g.

iface.legendInterface().setLayerVisible(layer, False)

# becomes
QgsProject.instance().layerTreeRoot().findLayer(layer.id()).setItemVisibilityChecked(False)

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsLayerTreeNode.setItemVisibilityChecked()

and QGIS breaking changes 'Removed Classes' section

and this GIS Stack Exchange answer

To fix: replace .legendInterface() with .layerTreeView()

e.g.

iface.legendInterface().selectedLayers()

# becomes
iface.layerTreeView().selectedLayers()

See also: QGIS Python API (PyQGIS) doc for qgis.gui.QgsLayerTreeView.selectedLayers()

and QGIS breaking changes for QgsGeometry

To fix: replace .fromPoint() with .fromPointXY()

e.g.

QgsGeometry.fromPoint(qgs_point)

# becomes
QgsGeometry.fromPointXY(qgs_point_xy)

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsGeometry.fromPointXY()

and QGIS breaking changes for QgsGeometry

To fix: replace .exportToWkt() with .asWkt()

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsGeometry.asWkt()

and QGIS breaking changes for QgsGeometry

To fix: refer to this GIS Stack Exchange answer

and QGIS breaking changes for QgsPalLayerSettings

To fix: replace .setDisplayField(html) with .setMapTipTemplate(html)

e.g.

layer.setDisplayField(html)

# becomes
layer.setMapTipTemplate(html)

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsVectorLayer.setMapTipTemplate()

and QGIS breaking changes for QgsVectorLayer

To fix: replace .setLayerTransparancy(percentage) with .setOpacity(percentage / 100)

e.g.

layer.setLayerTransparency(60)

# becomes
layer.setOpacity(0.6)

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsVectorLayer.setOpacity()

and QGIS breaking changes for QgsVectorLayer

To fix: replace .rendererV2().symbols() with .renderer().symbol() and access the symbol directly

e.g.

symbols = layer.rendererV2().symbols()
symbol = symbols[0]

# becomes
symbol = layer.renderer().symbol()

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsSingleSymbolRenderer.symbol()

and QGIS Python API (PyQGIS) doc for qgis.core.QgsVectorLayer.setRenderer()

and QGIS breaking changes 'Renamed Methods' section

To fix: replace .setRendererV2(renderer) with .setRenderer(renderer)

e.g.

layer.setRendererV2(renderer)

# becomes
layer.setRenderer(renderer)

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsVectorLayer.setRenderer()

To fix: replace QGis.xxx with Qgis.xxx (note change in case from QG to Qg)

e.g.

from qgis.core import QGis
QGis.xxx

# becomes 
from qgis.core import Qgis
Qgis.xxx

See also: QGIS API doc for Qgis

and QGIS breaking changes for Qgis

  overload 1: too many arguments
  overload 2: not enough arguments
  overload 3: not enough arguments
  overload 4: not enough arguments
  overload 5: argument 1 has unexpected type 'QgsCoordinateReferenceSystem'

To fix: add QgsProject.instance() to QgsCoordinateTransform() call

e.g.

xform = QgsCoordinateTransform(src_crs, dest_crs)


# becomes
xform = QgsCoordinateTransform(src_crs, dest_crs, QgsProject.instance())

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsCoordinateTransform

and QGIS breaking changes for QgsCoordinateTransform

To fix: replace thePromptToSaveFlag with promptToSaveFlag

e.g.

iface.newProject(thePromptToSaveFlag=False)

# becomes
iface.newProject(promptToSaveFlag=False)

See also: QGIS Python API (PyQGIS) doc for qgis.gui.QgisInterface.newProject()

overload 1: argument 1 has unexpected type 'QFileInfo'
overload 2: too many arguments

To fix: remove QFileInfo and just pass string to QgsProject.read()

e.g.

project.read(QFileInfo(project_file))

# becomes
project.read(project_file)

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsProject.read()

and QGIS breaking changes for QgsProject

  overload 1: too many arguments
  overload 2: argument 1 has unexpected type 'QgsPoint'
  overload 3: argument 1 has unexpected type 'QgsPoint'
  overload 4: argument 1 has unexpected type 'QgsPoint'
  overload 5: argument 1 has unexpected type 'QgsPoint'

To fix: replace instances of QgsPoint with QgsPointXY

e.g.

QgsRectangle(QgsPoint(x_min, y_min), QgsPoint(x_max, y_max)))

# becomes
QgsRectangle(QgsPointXY(x_min, y_min), QgsPointXY(x_max, y_max)))

See also: QGIS Python API (PyQGIS) doc for qgis.core.QgsRectangle

and QGIS breaking changes 'Renamed Classes' section

To fix: replace instances of QgsPoint with QgsPointXY

e.g.

QgsPoint(..., ...)

# becomes
QgsPointXY(..., ...)

See also: QGIS Python API (PyQGIS) doc for qgis.gui.QgsRubberBand.addPoint()

and QGIS breaking changes 'Renamed Classes' section

If the message you are getting when trying to migrate your plugin is not on this page, you can refer to:

• The QGIS API 'Backwards Incompatible Changes' (a.k.a. API Break) page
• The QGIS 'Plugin migration to QGIS 3' page
• GIS for Thought's useful walk-through on 'Updating A Plugin From QGIS 2 to QGIS 3'
• Stack Exchange's essential 'Geographical Information Systems' Q&A site

I also found the First Aid QGIS plugin quite useful throughout the process, see:
First Aid's GitHub page and their blog post introducing it for more details.

Contributions to improve this document are welcome, whether in the form or corrections, changes, additional entries, or anything else you see fit. Please submit a pull request.

common-qgis-2-to-3-plugin-fixes is licensed under a Creative Commons Attribution 4.0 International License.