I am using the GRASS Python Scripting Library on Ubuntu 14.04 with the packages available from the Universe repository. I have the following block of code:

            grass.mapcalc("$weightLine = $seg_rast * $cost",
                        weightLine = "weightLine",
                        seg_rast = "seg_rast",
                        cost = "cost")
            self.logger.debug("Exception running mapcalc: " + 

        kwargs = {}
        kwargs['stdout'] = grass.PIPE
        kwargs['stderr'] = grass.PIPE
        ps = grass.start_command("r.sum", rast="weightLine", **kwargs)
        mesg = ps.communicate()
        self.logger.info("r.sum: %s", str(mesg))

Which is logging the following:

[2014-06-03 09:57:35,532] DEBUG: Exception running mapcalc: (<type 'exceptions.SystemExit'>, SystemExit(1,), <traceback object at 0x7ffcbab1de18>)
[2014-06-03 09:57:35,538] INFO: r.sum: ('', 'ERROR: Raster map <weightLine> not found\n')

The employment of sys.exit() is not very helpful, even though it can be worked around. Without the except: block it exits my script and any other processes that may have invoked it. Note also that except Exception, e: does not catch exceptions.SystemExit.

But worst of all, the mapcalc instruction is not succeeding. It is either failing - in which case an error message is lacking - or it is simply not being executed.

How can I know the cause of this?

Update I: I have tried to run grass.mapcalc in different ways:

grass.mapcalc("weightLine = seg_rast * cost")


weightLine = "weightLine"
seg_rast = "seg_rast"
cost = "cost"            
grass.mapcalc("%s = %s * %s" % (weightLine, seg_rast, cost))

But the result is always the same: exceptions.SystemExit and the command is not executed.

  • 1
    You need to use %s in the mapcalc function (it is Python, not shell). See e.g. this randomly selected example: trac.osgeo.org/grass/browser/grass/trunk/scripts/i.pansharpen/… – markusN Jun 3 '14 at 19:22
  • Hi Markus, if I try with grass.mapcalc("weightLine = seg_rast * cost") I get the exact same result: the SystemExit exception and no map calculated. – Luís de Sousa Jun 4 '14 at 5:55
  • I think what Markus meant is something like: grass.mapcalc('%s = %s * %s" % (weightLine, seg_rast, cost)) – Micha Jun 4 '14 at 7:26
  • Hi Micha, I already tried this too. I do not think this problem is related to string formatting, in that case I would probably be receiving a different exception or an error message. – Luís de Sousa Jun 4 '14 at 7:30
  • I guess you have already confirmed that the command directly in GRASS works as expected? – Micha Jun 4 '14 at 9:14

Here a working grass.mapcalc() example based on your map names:

import grass.script as grass

weightLine = "weightLine"
seg_rast = "seg_rast"
cost = "cost"
grass.mapcalc("%s = %s * %s" % (weightLine, seg_rast, cost))

In case of errors, check if the input maps are existing.

| improve this answer | |
  • Hi again Markus. I do not get any error messages up to the r.sun command. The question is, if any of datasets is missing, shouldn't I be receiving an error message instead of exceptions.SystemExit? – Luís de Sousa Jun 5 '14 at 6:48
  • Prior to the mapcalc command the seg dataset is being imported with: grass.read_command("v.in.ogr", flags="o", output="seg", input="seg.gml"). I find out this command is failing, but I am getting no exception, nor any error message. I would expect: a) v.in.org to return an error message in case it fails; b) mapcalc not invoking sys.exit() but rather to return an error message too. – Luís de Sousa Jun 5 '14 at 8:29
  • 1
    Hi, you need to check the return code of every process you run (in case of *_command functions). I actually think that the GRASS API should be improved to support what you are asking for (trac.osgeo.org/grass/ticket/2326). mapcalc function exits because it is consistent with the behavior of GRASS C library. However, you can change this behavior by calling grass.script.core.set_raise_on_error(True) at the beginning of your script then ScriptError will be raised. – wenzeslaus Jun 5 '14 at 19:59

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