Are your jobs running on windows or are they running on unix?
When you say that you want to execute the command
are you expecting this:
exec.py -a a_value -b "b_mod -b_op1 b_op1_value"
Arguments #1
arg1: -a
arg2: a_value
arg3: -b
arg4: "b_mod -b_op1 b_op1_value"
#1 above should result from this:
arguments = "-a a_value -b '"b_mod -b_op1 b_op1_value"'"