CODEWITHSUNDEEP
pythonlistrecursionappendpython-2.x
aerin
aerin

Reputation: 22634

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

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?

Upvotes: 1

Views: 789

Answers (2)

Will Da Silva
Will Da Silva

Reputation: 7040

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)

Upvotes: 2

Amber
Amber

Reputation: 526593

This is due to the following line:

arr = PrintAllPerms(n-1,arr,str_)

Your PrintAllPerms function doesn't return anything if it takes the else path, and thus is treated as if it's returning None. So arr gets set to None.

Upvotes: 3

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