[GRASS-dev] Question about *.mapcalc_start in Python scripting library

[Moritz Lennert]

Le Fri, 15 Mar 2019 13:15:16 +0000,
Michael Barton <Michael.Barton at asu.edu> a écrit :

> I've been using *.mapcalc_start to do some simple parallel processing
> in the i.pansharpen module and in r.landscape.evol. My understanding
> previously has been that *.mapcalc_start returns a Popen object that
> can be directed to wait to finish (like *.start_command), allowing
> multiple mapcalc operations to be initiated on multiple cores.
> 
> But when running r.landscape.evol on another computer, it is acting
> like mapcalc is not waiting to finish in spite of the *.wait()
> commands.
> 
> The description in the programmer's manual is not clear to me on this
> (see below). In one place, it says it does not wait to finish and in
> the example it says it does wait.


AFAIU, the function mapcalc_start() does not wait, i.e. you can go on
with your program while the mapcalc call is still ongoing. If you want
to wait, you have to explicitely call the function wait() of the Popen
object as is done in the example.

Moritz

> 
> Can anyone clarify this?
> 
> Thanks
> Michael
> 
> ==== from programmer's manual ====
> 
> script.raster.mapcalc_start(exp, quiet=False, verbose=False,
> overwrite=False, seed=None, env=None, **kwargs)[source] Interface to
> r.mapcalc, doesn’t wait for it to finish, returns Popen object.
> 
> >>> output = 'newele'
> >>> input = 'elevation'
> >>> expr1 = '"%s" = "%s" * 10' % (output, input)
> >>> expr2 = '...'   # etc.
> >>> # launch the jobs:
> >>> p1 = mapcalc_start(expr1)
> >>> p2 = mapcalc_start(expr2)  
> ...
> >>> # wait for them to finish:
> >>> p1.wait()  
> 0
> >>> p2.wait()  
> 1
> ____________________
> C. Michael Barton
> Director, Center for Social Dynamics & Complexity
> Professor of Anthropology, School of Human Evolution & Social Change
> Head, Graduate Faculty in Complex Adaptive Systems Science
> Arizona State University
> 
> voice:  480-965-6262 (SHESC), 480-965-8130/727-9746 (CSDC)
> fax: 480-965-7671 (SHESC),  480-727-0709 (CSDC)
> www: http://www.public.asu.edu/~cmbarton, http://csdc.asu.edu
> 
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