def pstdev(data, mu=None):
 var = pvariance(data, mu)
 try:
  return var.sqrt()
 except AttributeError:
  return math.sqrt(var)

def pstdev ( data , mu = None ) : var = pvariance ( data , mu ) try : return var . sqrt ( ) except AttributeError : return math . sqrt ( var )
return the square root of the population variance .