PyQGIS standalone script error message for gdal_merge: "Incorrect parameter value for INPUT"

Question

In my standalone PyQGIS script I would like to merge two raster layer. "zero" and "raster". Both variables are definitly QgsRasterLayer objects. This is the function:

def merge(self):
    zero = self.setToZero()
    raster = self.getRasterLayer()
    outDir = os.path.join(QgsProject().instance().homePath(), "Verschmelzung.tiff")
    param = {"input": [zero, raster],
             "output": outDir}
    merge = processing.run('gdal:merge', param)
    rlayer = QgsRasterLayer(merge['output'], 'mergedRaster')
    return rlayer

An that is the error message:

_core.QgsProcessingException: Unable to execute algorithm Incorrect parameter value for INPUT  Traceback (most recent call last):   File "C:/Users/Jan/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\gebeude_betroffenheit\Gebeude_Betroffenheit.py", line 347, in run
    self.zonalStatistics()   File "C:/Users/Jan/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\gebeude_betroffenheit\Gebeude_Betroffenheit.py", line 315, in zonalStatistics
    raster = self.focalStatistics()   File "C:/Users/Jan/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\gebeude_betroffenheit\Gebeude_Betroffenheit.py", line 297, in focalStatistics
    raster = self.merge()   File "C:/Users/Jan/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\gebeude_betroffenheit\Gebeude_Betroffenheit.py", line 292, in merge
    merge = processing.run('gdal:merge', param)   File "C:/PROGRA~1/QGIS3~1.4/apps/qgis-ltr/./python/plugins\processing\tools\general.py", line 105, in run
    return Processing.runAlgorithm(algOrName, parameters, onFinish, feedback, context)   File "C:/PROGRA~1/QGIS3~1.4/apps/qgis-ltr/./python/plugins\processing\core\Processing.py", line 139, in runAlgorithm
    raise QgsProcessingException(msg)
_core.QgsProcessingException:
    Unable to execute algorithm Incorrect parameter value for INPUT

According to this link it wants a list of rasters, and that is what I'm giving no.

I don't see the problem…

I'm using QGIS 3.4

Answer 1

All letters of keys (input, output etc.) in params have to contain just capital letters.

Your script will work, if you convert {"input": [zero, raster], "output": outDir} into {"INPUT": [zero, raster], "OUTPUT": outDir}. At least, you don't encounter that error.