import math


def pstdev(lst):
    """Return the population standard deviation of a float list."""
    if len(lst) == 0:
        return None
    else:
        mean_squares = sum(list(map(lambda x: x**2, lst)))/float(len(lst))
        square_mean = (sum(lst)/float(len(lst)))**2
        return math.sqrt(float(mean_squares) - (square_mean))