def closest(position, positions):
    x0, y0 = position
    dbest, ibest = None, None
    for i, (x, y) in enumerate(positions):
        d = (x - x0) ** 2 + (y - y0) ** 2
        if dbest is None or d < dbest:
            dbest, ibest = d, i
    return ibest
import random

positions = [(random.random(), random.random()) for _ in xrange(10000000)]

%timeit closest((.5, .5), positions)

%pylab

positions = rand(10000000, 2)

type(positions)

positions.ndim, positions.shape

x, y = positions[:,0], positions[:,1]

distances = (x - .5) ** 2 + (y - .5) ** 2

%timeit exec In[9]

%timeit ibest = distances.argmin()