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 / float(len(grades))

return average

def grades_variance(grades):

average = grades_average(grades)

variance = 0

for n in grades:

variance = variance + (average - n)**2

return float(variance) / len(grades)

def grades_std_deviation(grades):

variance = grades_variance(grades)

return variance **0.5

print grades_std_deviation(grades)

why is it showing "grades_std_deviation(0.0) raise an error: 'float' object isn't iterable" ??

how can I fixed it><