grass - Using r.mapcalculator and Python

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.