Error: Could not load source layer for INPUT

Question

Very simple piece of code. I'm trying to run this as a stand alone Python script, not in the console. Keeps giving the error "Could not load source layer for INPUT", shape not found. But the os.path.isfile commands shows that the shapefile does exist. Driving me crazy this one problem. Been looking for solution for 2 days.

import sys, os
sys.path.extend([r'E:\sw_nt\QGIS_3.4\apps\qgis\python',r'E:\sw_nt\QGIS_3.4\apps\Python37\Lib\site-packages'])
os.environ['QT_QPA_PLATFORM_PLUGIN_PATH'] = r'E:\sw_nt\QGIS_3.4\apps\Qt5\plugins'
os.environ['PATH'] += r';E:\sw_nt\QGIS_3.4\apps\qgis\bin;E:\sw_nt\QGIS_3.4\apps\Qt5\bin'
sys.path.append(r'E:\sw_nt\QGIS_3.4\apps\qgis\python')
sys.path.append(r'E:\sw_nt\QGIS_3.4\apps\qgis\python\plugins')
sys.path.append(r'E:\sw_nt\QGIS_3.4\apps\qgis\python\plugins\processing')
from qgis.core import (
 QgsApplication, 
 QgsProcessingFeedback, 
 QgsVectorLayer
)
from qgis.analysis import QgsNativeAlgorithms
QgsApplication.setPrefixPath('/usr', True)
qgs = QgsApplication([], False)
qgs.initQgis()
import processing
from processing.core.Processing import Processing
Processing.initialize()
QgsApplication.processingRegistry().addProvider(QgsNativeAlgorithms())
feedback = QgsProcessingFeedback()
rivers = r'W:\srm\wml\Workarea\mamcgirr\QGIS_learning_projects\ne_10m_rivers_lake_centerlines.shp'
output = r'W:\srm\wml\Workarea\mamcgirr\QGIS_learning_projects\bcgw_test_data_as_shapes\clean_polys\danube3.shp'
expression = "name LIKE '%Danube%'"
print (os.path.isfile(rivers))
danube = processing.run(
 'native:extractbyexpression',
 {'INPUT': rivers, 'EXPRESSION': expression, 'OUTPUT': output},
 feedback=feedback
 )['OUTPUT']
print(danube)

• strythe: Have you tried using forward slashes instead of back slash in the path?
• Mark McGirr: I tired this. Exact same result.
• Mark McGirr: Perhaps there is some library not being loaded. I tried loading the 'modified script' another person submitted, but it looked exactly the same as the script I loaded.

Answer 1

I met the same problem as You. I'm new to Python, but I found it worked to use a QgsVecotrLayer instance instead of the layer path. Change the defination of rivers to

rivers = QgsVectorLayer("r'W:\srm\wml\Workarea\mamcgirr\QGIS_learning_projects\ne_10m_rivers_lake_centerlines.shp'","rivers","ogr")

By the way, if you get the direct solution please share with me.

Answer 2

I found the solution. My mistake was that after adding the raster or vector layer when I was planning to use it for further process, I selected it from the drop-down list, while you should change the state to model input and then select the layer. When this message appears, if you look at the model flowchart, the source layer will not be connected to the next component. While if you do as I aforementioned, the problem would be solved. I was engaged by this problem and error for many days and now I found my mistake. Look at the attached pictures.

enter image description here