Hello, I was studying Python Code Cahallenges :Lists

The question is below:

Write a function named

`append_sum`

that has one parameter β a list named named`lst`

.The function should add the last two elements of

`lst`

together and append the result to`lst`

. It should do this process > three times and then return`lst`

.For example, if

`lst`

started as`[1, 1, 2]`

, the final result should be`[1, 1, 2, 3, 5, 8]`

.

My solution code is below:

```
def append_sum(lst):
for i in range(3):
a = int(lst[-2]) + int(lst[-1])
b = lst.append(a)
return b
```

But this code snippet returns βNoneβ. Why?