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?

UPDATE: Note, that 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

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__()


  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
    if proj != new_proj:
      sys.exit('CRSs do not match!')
  • 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. – Curlew Dec 29 '13 at 0:56
  • @Curlew, I added relevant part of code. – SS_Rebelious Dec 29 '13 at 11:59
  • first thing, osgeo.gdal is for pure Python scripting:

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

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


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():

and the script terminate by himself.

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


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)


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

Result in the Python console:

enter image description here

  • 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. – SS_Rebelious Dec 29 '13 at 20:20
  • 1
    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. – gene Dec 29 '13 at 20:45
  • ok, good to know. BTW it was't me who down-voted your answer. – SS_Rebelious Dec 29 '13 at 21:20
  • 1
    It is a problem of PyQT4, look at simple IPython example raises exception on sys.exit() – gene Dec 29 '13 at 21:38
  • 1
    sys.exit()kills Python and QGIS. I found another solution: embedding the code in a function and using sys.exitfunc()(see above) – gene Dec 29 '13 at 23:08

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
    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


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