I've been trying to write a script creating binary maps:

a=raw_input ("enter a threshold") 
grass.run_command('r.mapcalculator', formula="**'base_map'>=b**", outfile='map'+str(a), overwrite=True) 

When I'm trying to execute it there's an error message :

Invalid map
Parse error 

Obviously, when the relevant part of the code is changed (e.g. formula="'base_map'>=**0.7**") everything is working fine. However, my goal was to write a script creating a map automatically when the "a" (the threshold) is provided.

  • I'm really not an expert, but is it possible that the formula is passed as such to Grass: "base_map >= b" (where you actually would like "base_map >= value_of_b"). Maybe you should somehow escape "b"? Just my 2 cents... Commented Mar 17, 2012 at 10:06
  • this is indeed a crucial question. how to "tell" grass to treat "b" as a value provided by the user (a=raw_input, b=float(a)) rather than a map (which obviously does not exist? as you can see I'm not an expert either
    – Dawid
    Commented Mar 17, 2012 at 10:21
  • Have you already made som tries such as "base_map >= " b? I haven't ever found any extensive documentation on the Grass API for python, so my first guess would be to try a few options like this... But maybe someone more clever than me has a more clever idea! Commented Mar 17, 2012 at 11:08
  • This is a FAQ. May I suggest printing out the value of formula? Specifically, display the value of the expression "**'base_map'>=b**". I think the problem and its solution will then immediately be clear.
    – whuber
    Commented Mar 17, 2012 at 14:51

2 Answers 2


The GRASS Python library uses the command grass.mapcalc, which is more along the lines of what you're looking for. It takes a string that is the same as the string you'd enter manually at the command line. Therefore if I'd enter the following string at the command line in GRASS:

r.mapcalc 'new_map = old_map - older_map * 32'

I could use the following line in Python to do the same thing:

import grass.script as grass
grass.mapcalc('new_map = old_map - older_map * 32')

So if you're wanting to construct a command for r.mapcalc in Python, try putting it together as a string. Here's an adapted version of your code:

a = raw_input("Enter a threshold: ") 
formula = 'map' + a + ' = base_map >= ' + a

Apparently the following works:

grass.mapcalc("$x=$A>=$b", x='output_map'+str(a), A='base_map', b=b)

in older versions of GRASS, the output will be saved as 'expression' so it is wise to copy and rename it before running the code again.

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