How can I get the path for an input layer in a QGIS 3 script?

Question

Writing scripts in previous versions of QGIS, I would get the layer path to pass to algorithms using processing.getObject(layer). In QGIS 3, the scripting system has been rewritten, and I now define a 'source' for input layers like this:

SPECIES_POINTS = 'SPECIES_POINTS' # INPUT = 'INPUT'
[...]
speciesPointSource = self.parameterAsSource(
 parameters,
 self.SPECIES_POINTS,
 context
 )

I then try to pass it to an algorithm:

pointsReprojected = processing.run("native:reprojectlayer", {
 'INPUT': speciesPointSource,
 'TARGET_CRS': CRS,
 'OUTPUT' : speciesExtent_id
 }, context=context, feedback=feedback)['OUTPUT']

but when I try to run this, I get an error:

Unable to execute algorithm
Could not load source layer for INPUT: invalid value
Traceback (most recent call last):
File "<string>", line 191, in processAlgorithm
File "C:/OSGEO4~1/apps/qgis/./python/plugins\processing\tools\general.py", line 96, in run
return Processing.runAlgorithm(algOrName, parameters, onFinish, feedback, context)
File "C:/OSGEO4~1/apps/qgis/./python/plugins\processing\core\Processing.py", line 142, in runAlgorithm
raise QgsProcessingException(msg)
_core.QgsProcessingException: Unable to execute algorithm
Could not load source layer for INPUT: invalid value

I can see that speciesPointSource has type QgsFeatureSource, so I'm not surprised this doesn't work, but I can't see any attributes that would give me the file path for the layer. I have also tried self.SPECIES_POINTS but then I get the error Could not load source layer for INPUT: SPECIES_POINTS not found. What should I be passing as 'INPUT'?

Answer 1

The input parameters can be accessed as a dictionary object, so parameters['SPECIES_POINTS'] will give you the path to the input layer. The call to the algorithm would then be:

pointsReprojected = processing.run("native:reprojectlayer", {
 'INPUT': parameters['SPECIES_POINTS'],
 'TARGET_CRS': CRS,
 'OUTPUT' : speciesExtent_id
 }, context=context, feedback=feedback)['OUTPUT']

Note that if you are only going to be passing the input object to an algorithm without doing anything with it, it's not necessary to define it as a source. The speciesPointSource = self.parameterAsSource(...) section is not needed.

• spatialthoughts: Thank you so much for this. After hours of trying, I figured that must be the only way and landed here to see the confirmation.