[GRASS-dev] grass.mapcalc() python wrapper using grass.start_command()

[Hamish]

Hamish wrote:
> in making some of the python modules multithreaded, it would
> be useful to have a version of grass.mapcalc() which didn't
> wait until it was done, and returned the Popen object instead.
> 
> e.g. to process three r,g,b bands in parallel.
> 
> I just added an experimental grass.mapcalc_start() fn to
> trunk/lib/python/raster.py,

an example for running 3 r.mapcalcs in parallel:

before:
  grass.mapcalc('%s.red = r#%s' % (out, in_R))
  grass.mapcalc('%s.grn = g#%s' % (out, in_G))
  grass.mapcalc('%s.blu = b#%s' % (out, in_B))

after:
  pr = grass.mapcalc_start('%s.red = r#%s' % (out, in_R))
  pg = grass.mapcalc_start('%s.grn = g#%s' % (out, in_G))
  pb = grass.mapcalc_start('%s.blu = b#%s' % (out, in_B))
  pr.wait()
  pg.wait()
  pb.wait()


since the parent process retains control of the pipes by default,
AFAIK p.communicate() is not needed here, errors and messages
just get propagated to the terminal as they occur.


Hamish