[GRASS-dev] r.mapcal and python

[Glynn Clements]

Moritz Lennert wrote:

> > I'm a python newbie so have some patience :-)
> > 
> > I'm trying to use r.mapcal with the grass.py function 'run_command' in 
> > this way:
> > 
> > e = "%s=%s-%s" % (out, dsm, dtm)
> > grass.run_command('r.mapcalc', expression = e)
> > 
> > However, the new raster in "%out" doesn't appear, and a new raster 
> > called 'expression' is created instead.
> > 
> > I also tried with:
> > 
> > grass.run_command('r.mapcalc', expression = "%s=%s-%s" % (out, dsm, dtm))
> 
> The expression= parameter only exists in GRASS7, but not in grass6.x.
> 
> So, r.mapcalc expression=k=1
> 
> will give you
> 
> in GRASS6.x: a map called 'expression' with the value 1
> 
> in GRASS7: a map called 'k' with the value 1
> 
> So everything depends on the version of GRASS you are running.
> 
> > with the same result. And also with:
> > 
> > grass.run_command('r.mapcalc', %s = "%s-%s" % (out, dsm, dtm))
> 
> No expert, but shouldn't this be:
> 
> grass.run_command('r.mapcalc', "%s = %s-%s" % (out, dsm, dtm))
> 
> [note the changed location of the opening quote]

In terms of Python syntax, the latter is correct.

However, grass.run_command() is intended for modules which use
G_parser(). In particular, you can only pass arguments which have the
form "name=value".

You can fudge it for the 6.x version of r.mapcalc with:

	grass.run_command('r.mapcalc', **{out: "%s-%s" % (dsm, dtm)})

But I would suggest either using subprocess.call():

	subprocess.call(['r.mapcalc', '%s = %s-%s' % (out, dsm, dtm)])

or passing the expression via stdin using grass.write_command():

	grass.write_command('r.mapcalc', stdin = '%s = %s-%s' % (out, dsm, dtm))

-- 
Glynn Clements <glynn at gclements.plus.com>