Why am I getting a list error on a python list (recursion)?

Question

I am trying to create custom permutations in a list (mainly to mess around with recursion in python). Right now I get this error when I run my code:

TypeError: 'NoneType' object is not iterable

Before I added in the list copying, I was getting this:

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

def findPermutations (size, max, curr_perm):
    if not curr_perm or len(curr_perm) < size:
        for i in range(1,max):
            new_arr = list(curr_perm)
            findPermutations(size, max, new_arr.append(i))
    else:
        return curr_perm


print(findPermutations(2,3,[]))

I was hoping to return a bunch or permutations. What am I doing wrong here?

NoneType means you're trying to access something that doesn't exist. In other languages known as null. Use a debugger and check the values. — Vulpex
– Vulpex, Commented Apr 1, 2019 at 16:24
new_arr.append(i) returns None. Append, then pass the list into the function — C.Nivs
– C.Nivs, Commented Apr 1, 2019 at 16:25

Venkata Muttineni · Accepted Answer · 2019-04-01 16:54:23Z

1

You need append the item list before calling recursive function. Below is the working code, Please let me know if you have any questions, i would be very happy to help you out.

def findPermutations (size, max, curr_perm):
    if not curr_perm or len(curr_perm) < size:
        for i in range(1,max):
            new_arr = list(curr_perm)
            new_arr.append(i)
            print(new_arr)
            findPermutations(size, max, new_arr)
    else:
        return curr_perm
findPermutations(2,3,[])

**Result:**
[1]
[1, 1]
[1, 2]
[2]
[2, 1]
[2, 2]

answered Apr 1, 2019 at 16:54

Venkata Muttineni

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4 Comments

JSells Over a year ago

Thanks! First: can you explain why having it in the function call didn't work? Second: how do I get the function itself to return the lists; it still returns None?

Venkata Muttineni Over a year ago

Here two things we need to consider: The list.append function does not return any value(but None), it just add the value to the list you are using to call that method and other one is python lists runs asynchronously until we specify. so in your case when you call new_arr.append(i) in function call, by the time, it considers list is empty before executing append in function call. For your second question, did you tried with my solution. I don't think you will see 'NONE' with my solution. let me know if you still have questions.

JSells Over a year ago

That makes sense, thanks. I have it working now, I didn't see that you had put in the print statement. Thanks again!

Venkata Muttineni Over a year ago

Glad you it helped. Could you please mark this as correct answer.

Wonka · Accepted Answer · 2019-04-01 16:29:10Z

0

To solve your code:

    new_arr.append(i)
    findPermutations(size, max, new_arr)

answered Apr 1, 2019 at 16:29

Wonka

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1 Comment

JSells Over a year ago

Thanks! Why do they need to be split up?

Rajat · Accepted Answer · 2020-08-12 08:45:57Z

0

The list.append() function returns None.

It does not return the new updated list. It simply modifies the existing list and returns None. Hence None is passed as the third parameter in the findPermutations() function in the subsequent recursive call.

The answer by @Wonka splits up the code into two parts. This allows the new_arr list to be updated first and then passed onto the function.

Using new_arr + [i] would be a better option here. This creates a new list and that is passed onto the next recursive function call.

edited Aug 12, 2020 at 8:45

answered Aug 12, 2020 at 8:29

Rajat

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