I think I already have valid output of this exercise.However, there's an error still.

error:

Oops, try again. double_list([0, 1]) returned None. Did you remember to return the result?

May anybody have a look for me, please? ><

Thanks.

```
n = [3, 5, 7]
def double_list(n):
for i in range(0, len(n)):
n[i] = n[i] * 2
print n
double_list(n)
#also it's unavailable to be written like below:
n = [3, 5, 7]
def double_list(n):
for i in range(0, len(n)):
n[i] = n[i] * 2
return print n
double_list(n)
#also I know it works as below:
n = [3, 5, 7]
def double_list(n):
for i in range(0, len(n)):
n[i] = n[i] * 2
return n
print (double_list(n))
#what's the different logic behind this three codes?
# Are we unable to calculate behind "Return" function?
```