Función que me de la mediana

def median(numbers):
  numbers.sort()
  if len(numbers) < 1:
    return None
  if len(numbers) %2 == 1:
    return numbers[((len(numbers)+1)/2)-1]
  if len(numbers) %2 == 0:
    return float(sum(numbers[(len(numbers)/2)-1:(len(numbers)/2)+1]))/2.0