Find out if a point is inside a geographic area

gistfile1.py

colorado = [(37.0, -102.03),
            (37.0, -109.03),
            (41.0, -109.03),
            (41.0, -102.03)]

denver = (39.44, -104.59)
cheyenne = (41.14, -104.80)


def poly_contains_point(poly, point):
    points = len(poly)
    inside = False

    p1 = poly[0]
    for i in range(points+1):
        p2 = poly[i % points]
        if min(p1[1], p2[1]) < point[1] <= max(p1[1], p2[1]):
            if point[0] <= max(p1[0], p2[0]):
                if p1[1] != p2[1]:
                    intercept = (point[1]-p1[1])*(p2[0]-p1[0])/(p2[1]-p1[1])+p1[0]
                if p1[0] == p2[0] or point[0] <= intercept:
                    inside = not inside
        p1 = p2
    return inside