[FIXED] AttributeError: 'NoneType' object has no attribute 'append' (recursion function)

Issue

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,[],"")

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<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?

Solution

You need to return arr in the else branch.

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]
        return arr

PrintAllPerms(2)

Answered By - Will Da Silva