How to batch run Mathematica code from Python?

Question

I have this notebook:

arg1 = "input.txt";
arg2 = "output.txt";
u1 = Import[arg1, "Table"];
u2 = u1(*extremely complicated and long chain of operations*);
Export[arg2, u2]

and I would like to run it in a Python program like this:

def run(arg1,arg2):
    my_mathematica_code(arg1,arg2)

for i in range(100):
    run("input{}".format(i), "output{}".format(i))

How can I do this? Preferably without GUI and several threads in parallel (I can use 4 cores in Mathematica).