Hey everybody, does anybody have an idea where the problem is ?

I get this error message:

in grades_std_deviation

TypeError: unsupported operand type(s) for ** or pow(): 'list' and 'float'

, but everything worked fine till now

```
import math
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(scores):
average = grades_average(scores)
variance = 0
for score in scores:
variance += ((average-score)**2)
variance= variance/ float(len(scores))
return variance
def grades_std_deviation(variance):
variance = float(variance **0.5)
return variance
variance=grades_variance(grades)
print print_grades(grades)
print ""
print grades_sum(grades)
print ""
print grades_average(grades)
print ""
print grades_variance(grades)
print ""
print grades_std_deviation(grades)
```