I'm trying to get all possible permutations of dice rolls. Below is my code.

```
def PrintAllPerms(n, arr, str_):
    if (n == 0):
        print str_
        arr.append(str_)
        return arr
    else:
        for i in ["1","2","3","4","5","6"]:
            str_ = str_ + i
            arr = PrintAllPerms(n-1,arr,str_)
            str_ = str_[:-1]

PrintAllPerms(2,[],"")

```

But I got following error after printing only this much.

```
PrintAllPerms(2,[],"")

11
12
13
14
15
16
21

<ipython-input-7-d03e70079ce2> in PrintAllPerms(n, arr, str_)
      2     if (n == 0):
      3         print str_
----> 4         arr.append(str_)
      5         return arr
      6     else:

AttributeError: 'NoneType' object has no attribute 'append'

```

Why does it print until 2,1 then?

And what is the correct to way to deal with this?