How to use processing in a PyQT plugin in QGis

Question

I am writing a QGis plugin with UI that should utilize certain QGIS processing tools when the user runs the plugin.

Now I tried a lot, searched a lot and came along quite many different approaches. First I want to succeed using QGis processes and second I want to make sure this survives the upcoming changes in QGis 3.0.

For now all I want is to use the process "gdalogr:cliprasterbyextent" and manged to use this one when opening the Python console and entering directly:

import processing
from qgis.core import QgsRasterLayer

raster = QgsRasterLayer('/tmp/test.tif', 'test.tif')
output = '/tmp/result.tif'
extent = '67.9986111111,97.5069444444,6.70138888889,37.1097222222'

processing.runalg('gdalogr:cliprasterbyextent', {'INPUT': raster, 'OUTPUT': output, 'PROJWIN': extent})

The same code in my Python plugin doesn't produce any output nor errors. It just runs through with not blocking the execution. I wasn't sure if the processing is properly callable from within my plugin? So I did a print("processing.runalg") and see that it is still a function. Yes, it is still a callable function and accessible from my Python plugin! I can also just use "processing.runalg('gdalogr:cliprasterbyextent')" and get the cliprasterbyextent help text when running my plugin. I also made sure the file exists and access rights are no obstacle.

I also saw examples where runalg is getting passed only arguments like:

processing.runalg('gdalogr:cliprasterbyextent', raster, '', extent, 5, 4, ...)

and I saw examples which first import and then initialize the Processing framework like this:

sys.path.append(QgsApplication.pluginPath())
from processing.core.Processing import Processing
Processing.initialize()

So far this all doesn't help me. I am lost and wonder why utilizing a simple process in my plugin seems so complicate. Why doesn't runalg throw any errors or block the execution for at least 1-3 seconds like when calling it from the QGis Console?

Could someone please indicate me what the right approach would be. I think, such cases should also be properly documented or an example guide should be in the QGis documentation/tutorials.

Answer 1

I can partly answer my own question!

The following works now also in my Python plugin: The reason was that I had a differed order of xmin/xmax/ymin/ymax for PROJWIN in the plugin code. Thus the module failed but did not throw any errors. Yes sometimes you don't see the wood for the trees ;-)

import processing
from qgis.core import QgsRasterLayer

raster = QgsRasterLayer('/tmp/test.tif', 'test.tif')
output = '/tmp/result.tif'
extent = '67.9986111111,97.5069444444,6.70138888889,37.1097222222'

processing.runalg('gdalogr:cliprasterbyextent', {'INPUT': raster, 'OUTPUT': output, 'PROJWIN': extent})

BUT! I don't get the processing running as soon as I use it in an extra QThread. When using in the plugin thread, a simple "import processing" is all you need. As I want to parallelize my processing calls I use workers in a QThread. There are some examples here how to do processing in QThreads, to make your QGis plugin interruptible/stoppable with at least on QThread executin the plugin logic. I wonder if it is possible at all to use the processing module in a QThread or what approach would help me in this case.

Do I need to initialize the processing or a QGis application in every thread?