Terminating Python scripts in Processing framework

Question

I'm writing a Python script to use it in Processing framework. I need to cancel script execution if certain conditions are met (to save user's time). I tried to use sys.exit(arguments) command. The issue is that not only the script, but QGIS itself shuts down too. I also tried quit() but result is the same. How should I terminate script in Processing framework?

The following code is for reproduction purpose only. I don't need ad hoc solution for this particular case because I already have it. I need to know how to deal with this issue in general.

##Raster processing=group
##raster_1=raster
##raster_2=raster
from osgeo import gdal
import sys
from numpy import *
import ntpath
import re
import platform
from PyQt4 import QtGui
raster_1 = gdal.Open(raster_1)
raster_2 = gdal.Open(raster_2)
rasters_list = [raster_1, raster_2]
# create a message for the case when CRSs of rasters do not match
class WrongCRS(QtGui.QMessageBox):
 def __init__(self):
 super(WrongCRS, self).__init__()
 self.initUI()
 def initUI(self):
 self.warning(self, 'Oops!',
 "Rasters must have the same CRS!\n\nExecution cancelled!", QtGui.QMessageBox.Ok)
# check CRS
proj = None
for raster in rasters_list:
 new_proj = raster.GetProjection()
 if proj is None:
 proj = new_proj
 else:
 if proj != new_proj:
 WrongCRS()
 sys.exit('CRSs do not match!')
 else:
 continue

• Curlew: it might be helpful to get to know a bit more about your code(not only for reproduction). Are you trying to interrupt within a forloop or a try-catch phrase? I often end my script with writing none into all open files/variables.
• SS_Rebelious: @Curlew, I added relevant part of code.

Answer 1

• first thing, osgeo.gdal is for pure Python scripting:

  from osgeo import gdal 
  raster = gdal.Open('your.tif') 
  raster.GetProjection()
  

Everything else is a problem of osgeo, and not of PyQGIS, since you use 2 QGIS existing layers:

 ##raster_1=raster 
 ##raster_2=raster

you don't need it. To process the layers, the correct code is:

raster_1 = processing.getobject(raster_1) 
raster_2 = processing.getobject(raster_2)

an the crs is given by:

new_proj = raster.crs()

• second thing, you cannot load a raster in QGIS without projection (if not yet existing, it is chosen by QGIS, according to some criteria):

enter image description here

So your first condition if proj is None:, is unnecessary and your script become (you don't need a list with only two layers):

raster_1 = processing.getobject(raster_1)
raster_2 = processing.getobject(raster_2)
if raster_1.crs()==raster_2.crs():
 action
else:
 WrongCRS()

and the script terminate by himself.

---

in the last version of the processing module, it is now:

processing.getObjects()

---

For terminate the script, you can embed your script in a function and use sys.exitfunc() (I use the Python console to to see the results)

example:

import atexit
.....
def all_done():
 message = '- Rasters must have the same CRS!\n\nExecution cancelled!'
 print message 
atexit.register(all_done)
raster_1 = processing.getObject(raster_1)
raster_2 = processing.getObject(raster_2)
def main():
 if raster_1.crs() != raster_2.crs():
 sys.exitfunc()
 else:
 message = "ok"
 print message
 continue
main()

Result in the Python console:

enter image description here

• SS_Rebelious: FYI processing.getobjects() doesn't work any more. Thank you for advices on my code improvements, but I didn't ask for it and frankly they are pointless, cause I didn't ask for them and I had my reasons for that structure.Have you actually tested your own solution? After else: WrongCRS there are lines of code in my script and they will be executed if the script is not terminated. Finally where is the answer for my question that I expressed quite clear: how to terminate script in processing framework in general?. I was very close to down-vote your answer.
• gene: If you use the last version of processing, it is now processing.getObjects() (look at Create_tiling_from_vector_layer.py) but down-vote if you want.
• SS_Rebelious: ok, good to know. BTW it was't me who down-voted your answer.
• gene: It is a problem of PyQT4, look at simple IPython example raises exception on sys.exit()
• gene: sys.exit()kills Python and QGIS. I found another solution: embedding the code in a function and using sys.exitfunc()(see above)

Answer 2

It is impossible (for now at least) to terminate Processing python script and leave QGIS itself untouched. It was suggested by @gene to use sys.exitfunc() to terminate main function. However I find it more useful to use just return message to just end function normally. Here is pseudo code:

class ErrorMessage(...):
 .....
def checkFunction(parameter):
 if parameter == condition:
 return False
 else:
 return True
def jobFunction(...):
 ...
 if parameter_1 == condition_1:
 return False
 result = ...
 return result
def main(...):
 if not checkFunction(parameter):
 return ErrorMessage(...)
 result = jobFunction(...)
 if result == False:
 return ErrorMessage(...)
 return result
main(...)