First, longitude wraps from -180 to +180 at antimeridian, meaning distance calculations will fail there; second, and I'd say more importantly, one degree longitude length in meters differs a lot depending on latitude; meaning this library will be heavily biased towards longitudal neighbors when using it for locations far from equator.
http://geographiclib.sourceforge.net
There is a python implementation available as well. http://pypi.python.org/pypi/geographiclib
On it!
import math
from collections import namedtuple
def haversine_distance(origin, destination):
""" Haversine formula to calculate the distance between two lat/long points on a sphere """
radius = 6371 # FAA approved globe radius in km
dlat = math.radians(destination.lat-origin.lat)
dlon = math.radians(destination.lng-origin.lng)
a = math.sin(dlat/2) * math.sin(dlat/2) + math.cos(math.radians(origin.lat)) \
* math.cos(math.radians(destination.lat)) * math.sin(dlon/2) * math.sin(dlon/2)
c = 2 * math.atan2(math.sqrt(a), math.sqrt(1-a))
d = radius * c
# Return distance in km
return int(math.floor(d))
LatLng = namedtuple('LatLng', 'lat, lng')
origin = LatLng(51.507222, -0.1275) # London
destination = LatLng(37.966667, 23.716667) # Athens
print "Distance (km): %d" % haversine_distance(origin, destination)