[QGIS-Developer] Best practice for plugins including processing models

[Alister Hood]

Hi, I maintain an in-house plugin for our team of civil engineers, who
apart from myself are not very advanced GIS users.
It does things like set useful defaults and install a set of useful
plugins.  It also includes some processing models, which are loaded with
the code below, which seems to be the conventional way models are loaded in
publicly available plugins which ship processing models.  This code copies
the model files included in the plugin to the default folder for processing
models - somewhere like
%AppData%\Roaming\QGIS\QGIS3\profiles\default\processing\models

One of my colleagues did some QGIS training and created a processing model,
which coincidentally had the same filename as a model I created on the same
day, so his model got overwritten when my plugin was updated.  This is not
a problem for us as we are a small team and can be careful what we do going
forward.  But it highlights that the way plugin authors are loading model
files is what seems to me to be pretty poor practice, for two reasons.

Firstly models are available in qgis even when the plugin is unloaded or
uninstalled, which is probably not desirable.
It is trivial to implement an unloadModels function to rectify this, and I
have done so in my in-house plugin.  I'm wondering - should plugin authors
be encouraged to do the same?

Secondly, loading the plugin will silently overwrite any existing models
with the same filename, as occurred here.  Plugins generally contain models
with fairly special filenames, so hopefully this is unlikely to occur,
unless:
- someone produces a plugin that is a fork of another one, or
- someone ships a plugin with a model that is previously publicly available
somewhere else, or that was created by following an online tutorial, or
- a user edits a model provided by a plugin.
In any case, would it be a good idea to encourage plugin authors to load
models another way, without copying them into
%AppData%\Roaming\QGIS\QGIS3\profiles\default\processing\models?
I realise that to change an existing plugin there is the added complication
of what to do when updating from an old version - the only safeish way I
can think of to automatically clean up obsolete model files would be to
check for them and then use a checksum to verify whether they should be
deleted.
For what it's worth, I note that when a user tries to manually "add model
to toolbox", QGIS refuses if there is an existing model with the same
filename.

Thanks.


    def loadModels(self):
            '''Register models present in models folder of the plugin.'''
            try:
                iface.initializationCompleted.disconnect(self.loadModels)
            except:
                pass

            #QgsMessageLog.logMessage(self.tr('testing'), self.messageTag,
Qgis.Warning)
            modelsFiles = glob.glob(os.path.join(self.modelsPath,
'*.model3'))

            for modelFileName in modelsFiles:
                alg = QgsProcessingModelAlgorithm()
                if not alg.fromFile(modelFileName):
                    QgsMessageLog.logMessage(self.tr('Not well formed
model: {}'.format(modelFileName)), self.messageTag, Qgis.Warning)
                    continue

                destFilename =
os.path.join(ModelerUtils.modelsFolders()[0],
os.path.basename(modelFileName))
                try:
                    if os.path.exists(destFilename):
                        os.remove(destFilename)

                    if isWindows():
                        copyfile(modelFileName, destFilename)
                    else:
                        os.symlink(modelFileName, destFilename)
                except Exception as ex:
                    QgsMessageLog.logMessage(self.tr('Failed to install
model: {} - {}'.format(modelFileName, str(ex))), self.messageTag,
Qgis.Warning)
                    continue


QgsApplication.processingRegistry().providerById('model').refreshAlgorithms()
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