Using r.mapcalculator and Python

Question

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

a=raw_input ("enter a threshold") 
b=float(a) 
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.

Answer 1

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
grass.mapcalc(formula) 

Answer 2

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.