Python plugin API changes from 1.8 to 2.0

This page summarizes the changes needed in migrating QGIS python plugins from the 1.8 API to the 2.0 API. The version 2.0 API has many breaking changes which plugins need to account for.

It is recommended to create a new version of plugins for 2.0, rather than include conditional code to run in both 2.0 and 1.8 in all but the simplest of plugins. Note that both QGIS and the plugin repository distinguish between 1.8 and 2.0 version plugins. QGIS stores 2.0 plugins in a different location (~/.qgis2/python/plugins) to version 1.8, so a user can have both versions installed alongside one another. The repository distinguishes different versions using the plugin metadata.

Also see this page for changes in API not strictly related to Python API_changes_for_version_20.

Please add to this if you find something missing!

SIP API upgrade

The SIP API manages the mapping between python and C++/Qt objects. This has been upgraded to version 2. The most significant impact of this is that there is a much tighter mapping between Qt data types and python data types - QVariant and QString are removed. Also the "old style" signal and slot format is no longer available.

QVariant removed

The "QVariant" type doesn't exist anymore so any methods returning "QVariant" will be auto converted to Python types. You no longer need to convert the return type using the "toXXXX" methods.

Remove all:

 1   toString()
 2   toList()
 3   toInt()
 4   toFloat()
 5   toStringList()
 6   toByteArray()
 7   toPyObject()
 8   QVariant(..)
 9   QString(...)

Note that the autoconversion to a Python type is based on the type of the QVariant, which may not be the same as the type returned by a toXXX conversion. So new code may need to explicitly set the python type. Note also that some of the toXXX functions return a tuple of (type, valid) to specify whether the conversion is successful. For example:

Before:


value,ok = variantValue.toDouble()
if not ok:
    handleError()

After:


     # If you are really confident the variant is the type you expect, just use it
    value = variantValue 

    # Best option to ensure value has the same type as in original code
    value = float(variantValue)  

    # To handle conversion errors
    try: 
        value=float(variantValue)
    except:
        handleError()

Note: If you do not explicitly set the python type, then you can introduce some subtle errors where the following code assumes a specific type of value. For example value/10 will give a different result depending on whether value is an integer or a float.

QSettings return type

The type of QSettings return values is specified in the QSettings.value() call. More info: http://pyqt.sourceforge.net/Docs/PyQt4/pyqt_qsettings.html.

Before:


  settings.value(“/yourboolsetting”, True).toBool()
  settings.value(“/yourintsetting”, 10).toInt()[0]
  settings.value(“/yourintsetting”).toByteArray()

After:


  settings.value(“/yourboolsetting”, True, type=bool)
  settings.value(“/yourintsetting”, 10, type=int)
  settings.value(“/yourintsetting”, QByteArray(), type=QByteArray)

Replace QString methods

"QString" no longer exists in the new QGIS API. Any methods that return a "QString" will be converted into a native Python "unicode". All QString methods need to be replaced with equivalent native string methods.

Before:


  yourstring.right(4)
  files.join(",")
  if yourstring.length() > 4:
  if yourstring.isEmpty()

After:


  yourstring[4:]
  ",".join(files)
  if len(yourstring) > 4
  if not yourstring

Replace QStringList with list

Before:


  mystrings = QStringList()

After:


  mystrings = []

Remove QVariant calls

The "QVariant" also doesn't exist as an instantiated type anymore - any methods returning "QVariant" will be auto converted to Python types. However "QVariant" can still be used to access it's enum values e.g. "QVariant.Int" can set be used.

Before:


  myvalue = QVariant(10)
  myvalue = QVariant("Hello World")

After:


  myvalue = 10
  myvalue = "Hello World" 

Note that Null QVariant values (ie values for which QVariant.IsNull() returns True) are not mapped to the python None value as you might expect.
Instead they return a QPyNullVariant value. This preserves the type information of the null object.

Replace QList methods with python list function

Before:


  if files.isEmpty()
  files.count()

After:


  if not files
  len(files)

Replace signals with new style signals and connections

Emitting before:


  self.emit(SIGNAL("valuesChanged(const QStringList &)"), self.getArguments())

After:


  class Test():
    valuesChanged = QtCore.pyqtSignal(list)

    def yourmethod():
      self.valuesChanged.emit(self.getArguments)

Connecting before:


  QObject.connect(self.iface,SIGNAL('projectRead ()'),self.readSettings) 

After:


  self.iface.projectRead.connect(self.readSettings)

Vector layer API changes

QgsFeatureRequest replaces select(), featureAtId()

In QGIS 1.8 features are selected from a vector layer by using QgsVectorLayer.select() and then loop over provider.nextFeature(). In QGIS 2.0 the selection is defined by a QgsFeatureRequest object and features are retrieved using a python iterator created by QgsVectorLayer.getFeatures(QgsFeatureRequest). The QgsFeatureRequest object is only required to add selection criteria to the request - otherwise it can be omitted and all features will be returned.
In the same way, use QgsFeatureRequest to change use of featureAtId() layer method.

Before:


    layer.select()
    f=QgsFeature()
    while layer.nextFeature(f):
       ....

After:


    for f in layer.getFeatures():
       ...

To add criteria to the selection you need to explicitly define a QgsFeatureRequest, for example


     request=QgsFeatureRequest()
     request.setFilterRect(areaOfInterest)

     for f in layer.getFeatures(request):
         ...

Other criteria and be set using setSubsetOfFields and setFlags...


     request.setSubsetOfFields([0,2])                  # Only return selected fields
     request.setSubsetOfFields(['name','id'],layer.pendingFields())  # More user friendly version
     request.setFlags( QgsFeatureRequest.NoGeometry )  # Don't return geometry objects

Getting/setting QgsFeature attributes simplified

Feature attributes can be get and set by index, for example

Before:


    index = layer.fieldNameIndex(fieldname)
    layer.select()
    f = QgsFeature()
    while layer.nextFeature(inFeat):
        fieldvalue=f.attributeMap()[index].toString())

After:


    for f in layer.getFeatures():
        fieldvalue=f[fieldname]

Feature attributes can also be set by index, for example:


    fields=layer.pendingFields()
    f = QgsFeature(fields)
    f['name']='Bruce'
    f['id']=42

NOTE: Do not use f=QgsFeature(layer.pendingFields()) - this will kill QGIS. The QgsFieldList returned by layer.pendingFields() must have at least the same lifetime as the QgsFeature.

Plugin repository and metadata changes

The plugin should include a metadata.txt file to upload to the repository. For example:


name=My Plugin
description=Does useful stuff
category=Plugins
version=1.0
experimental=False
qgisMinimumVersion=2.0
author=My name
email=myemail@somewhere.com
icon=./plugin.png

NOTE: There was a rumor you should include a qgisMaximumVersion tag to the metadata.txt. Normally you don't need to set it. For further details see Plugin_Compatibility

Plugin init.py file should contain only the classFactory() method, all other information is in metadata.txt. ALL other members should be deleted from init.py .

Making a plugin compatible with all QGIS versions

If you really want to do it, set qgisMinimumVersion to 1.0 and qgisMaximumVersion to 2.99 explicitly. This way you can overwrite the default maximum version that is floor(qgisMinimumVersion) + 0.99.

Testing for QGIS version


if QGis.QGIS_VERSION_INT < 10900:
    # Use the old API style
else:
    # Use the new API style

Testing for SIP api version (QGIS 2 uses SIP api v2)


import sip
if sip.getapi("QVariant") > 1:
    # Use the new API style
else:
    # Use the old API style

Note: This is not a recommend pattern except for small plugins as it will make your code complicated to read and maintain.