The solution below is one approach. Note that we must convert the provided arguments from string values representing angles in degrees to floats in radians.

import sys
import math

# Calculate the shortest distance between two points on a sphere provided on
# the command line as (longitude, latitude pairs). For example, Paris and Rome:
# 48.9,2.4 41.9,12.5

haversin = lambda alpha: math.sin(alpha/2)**2

def gc_distance(loc1, loc2, R=6378.1):
    """
    Return the great-circle distance between two points on a sphere of radius
    R, provided as (latitude, longitude) pairs, loc=(phi, lamdbda) in radians.

    """

    (phi1, lambda1), (phi2, lambda2) = loc1, loc2
    d = 2 * R * math.asin(math.sqrt(haversin(phi2-phi1)
                + math.cos(phi1)*math.cos(phi2)*haversin(lambda2-lambda1)))
    return d

loc1, loc2 = sys.argv[1:]
phi1, lambda1 = [math.radians(float(x)) for x in loc1.split(',')]
phi2, lambda2 = [math.radians(float(x)) for x in loc2.split(',')]

d = gc_distance((phi1, lambda1), (phi2, lambda2))
print('{} km'.format(int(d)))