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.

Germán Carrillo · Accepted Answer

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 FilesQGIS Wienaappsqgis"

Now your raster layer should be valid.

xunilk · Answer

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:

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)

Jeremy · Answer

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")

Loading raster layer in a standalone PyQGIS script

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