Python: Distance with Spherical Coordinates

gistfile1.py

def distance(lat1,lon1,lat2,lon2):
    import math
    # Earth Radius in KM 
    R = 6371
    dLat = math.radians(lat2-lat1);
    dLon = math.radians(lon2-lon1);
    lat1 = math.radians(lat1);
    lat2 = math.radians(lat2);
    a = math.sin(dLat/2) * math.sin(dLat/2) +  math.sin(dLon/2) * math.sin(dLon/2) * math.cos(lat1) * math.cos(lat2)
    c = 2 * math.atan2(math.sqrt(a), math.sqrt(1-a))
    d = R * c
    return d