Standard deviation

Hi,

While trying to get the deviation I keep getting a NameError: name 'variance' is not defined

Im out of ideas as to how this snippet could work as I expect it to return the deviation. Please assist. Here is the code:

```
def grades_std_deviation(variance):
variance = 0
for grade in grades:
variance = grades_variance(grades)
return variance** 0.5
print grades_std_deviation(variance)
```