How do I count the percentiles with python/numpy?
To calculate percentile with python you might be interested in the SciPy Stats package. It has the percentile function you're after and many other statistical goodies.
percentile() is available in numpy too.
import numpy as np
a = np.array([1,2,3,4,5])
p = np.percentile(a, 50)
print p
3.0