Python : Exam Statistics

grades = [100, 100, 90, 40, 80, 100, 85, 70, 90, 65, 90, 85, 50.5]

def print_grades(grades):
    for grade in grades:
        print grade

def grades_sum(grades):
    total = 0
    for grade in grades:
        total += grade
    return total
   
def grades_average(grades):
    sum_of_grades = grades_sum(grades)
    average = sum_of_grades / len(grades)
    return average
   

def grades_variance(grades, average):
    variance = 0
    for i in grades:
        variance += (average - i) ** 2
    return variance / len(grades)

def grades_std_deviation(variance):
    std_dev = variance ** 0.5
    return std_dev
   
print print_grades(grades)
print grades_sum(grades)
print grades_average(grades)
print grades_variance(grades,grades_average(grades))
print grades_std_deviation(grades_variance(grades, grades_average(grades)))