[GRASS-user] Re: Using temp files in Grass Script

[Glynn Clements]

Johannes Radinger wrote:

> I found that thread about temporary maps...
> 
> e.g in my case in my python script i use:
> 
>     #Convert river from vector to raster format
>     grass.run_command("v.to.rast",
>                       input = "river_gen",
>                       overwrite = True,
>                       output = "river_raster",
>                       use = "val",
>                       value = res)
>                       
>     #Thinning the rastarized river
>     grass.run_command("r.thin",
>                       input = "river_raster",
>                       overwrite = True,
>                       output = "river_raster_thin")
> 
> and here is the map river_raster only temporary for the thinning
> process and the river_raster_thin is used for further calculations.
> At the moment I have a g.remove to remove the river raster after
> the thinning process but should I handle that with temporary files/maps
> and if yes how is that exactly done in my python script?

1. In general, you should make an attempt at ensuring that temporary
map names won't conflict with any existing map name. The usual
approach is to include both ".tmp" and the PID in the map name, e.g.:

	import os
	...
	global tmpmap
	tmp_map = 'river_raster.tmp.%d' % os.getpid()

2. Deletion should ideally be handled using an exit handler, e.g.:

	import os
	import atexit
	import grass.script as grass

	tmp_map = None

	def cleanup():
	    if tmp_map:
		grass.run_command('g.remove', rast = tmp_map, quiet = True)

	def main():
	    global tmpmap
	    tmp_map = 'river_raster.tmp.%d' % os.getpid()
	    ...

	if __name__ == "__main__":
	    options, flags = grass.parser()
	    atexit.register(cleanup)
	    main()

Using an exit handler ensures that the temporary map gets removed
regardless of how the script terminates (e.g. if it terminates due to
an exception).

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