Loading raster layer in a standalone PyQGIS script

Question

I need to load a raster file from the provided file path. I wish to use it further in the Raster Calculator. I wrote a function which converts string (I assume it is a file path) to a raster object.

from qgis.core import QgsRasterLayer
from PyQt4.QtCore import QFileInfo
def StringToRaster(raster):
 # Check if string is provided
 if isinstance(raster,basestring):
 fileInfo = QFileInfo(raster)
 baseName = fileInfo.baseName()
 path = fileInfo.filePath()
 if (baseName and path):
 raster = QgsRasterLayer(path, baseName)
 if not raster.isValid():
 print "Layer failed to load!"
 return
 else:
 print "Unable to read basename and file path - Your string is probably invalid"
 return
 return raster

I have no idea why, but raster.isValid() always returns False. When I take exactly the same path which I provided to my function, I am able to add this layer to QGIS from the interface (from the menu Layer --> Add layer ---> Add raster layer).

My layer is a Float32 GeoTIFF with one band only.

enter image description here

• matandked: Example path: /home/mateusz/many times I tried/data/TTT/TRR___niskorozdzielczy.tif

Answer 1

If you keep getting non valid layers, even defining them correctly, you're probably missing the QGIS prefix definition in your script (in case that you work out of QGIS).

qgis_prefix="/usr" 
QgsApplication.setPrefixPath(qgis_prefix, True) 
QgsApplication.initQgis()

If you work on Windows, your qgis_prefix should be something like this:

qgis_prefix="C:\\Program Files\\QGIS Wiena\\apps\\qgis"

Now your raster layer should be valid.

• matandked: Thanks, it seems that you need to import additional modules? Error message: name 'QgsApplication' is not defined.
• Germán Carrillo: Right, you would need (at least) this line: from qgis.core import QgsApplication
• matandked: Great, now it works perfectly.

Answer 2

This code works in my Python Console:

from qgis.core import QgsRasterLayer
from PyQt4.QtCore import QFileInfo
def StringToRaster(raster):
 # Check if string is provided
 fileInfo = QFileInfo(raster)
 path = fileInfo.filePath()
 baseName = fileInfo.baseName()
 layer = QgsRasterLayer(path, baseName)
 QgsMapLayerRegistry.instance().addMapLayer(layer)
 if layer.isValid() is True:
 print "Layer was loaded successfully!"
 else:
 print "Unable to read basename and file path - Your string is probably invalid"
raster = '/home/zeito/Desktop/output2.tif'
StringToRaster(raster)

After running the code in the Python Console of QGIS:

result

For those who will work with QGIS 3:

from qgis.core import QgsRasterLayer
from PyQt5.QtCore import QFileInfo
def StringToRaster(raster):
 # Check if string is provided
 fileInfo = QFileInfo(raster)
 path = fileInfo.filePath()
 baseName = fileInfo.baseName()
 layer = QgsRasterLayer(path, baseName)
 QgsProject.instance().addMapLayer(layer)
 if layer.isValid() is True:
 print ("Layer was loaded successfully!")
 else:
 print ("Unable to read basename and file path - Your string is probably invalid")
raster = '/home/zeito/Desktop/output2.tif'
StringToRaster(raster)

• matandked: Thanks, but my code also works in Python console within QGIS. For some reason, when I give exactly the same path, isValid() returns False outside Python QGIS console (for example in iPython) and True in QGIS Python console.
• matandked: I added a picture to my question.

Answer 3

The QGIS documentation also proposes a second way, I found very helpful:

[R]aster layers can be loaded using the addRasterLayer function of the QgisInterface:

iface.addRasterLayer("/path/to/raster/file.tif", "layer name you like")