I know why this error arises, but don't know how to get around it. This is my code followed by the error message:

CODE:

```
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 x in grades:
total += x
return total
def grades_average(grades):
sum_of_grades = grades_sum(grades)
average = sum_of_grades / float(len(grades))
return average
def grades_variance(scores):
variance = 0
average = grades_average(scores)
for n in scores:
variance += (average - n) ** 2
result = variance / len(scores)
return result
def grades_std_deviation(variance):
return grades_variance(variance)** 0.5
print grades_std_deviation(grades)
```

ERROR:

Oops, try again. grades_std_deviation(4.0) raised an error: 'float' object is not iterable

Is there something I need to use other than a for loop? How am I supposed to handle an input for both a list and a float?