Recursive function: "NoneType object is not iterable"

Question

I was trying to understand a recursive function and in order to do that I felt like I needed to create a smaller example for myself. My own code, however, is throwing me a NoneType error. I don't really have a purpose with my function, I just want to know why the error occurs?

def recursive(seq):
    if not seq:
        return [seq]
    else:
        seq2=seq[1:]
            print('seq2= ',seq2)
            print('Type seq2 = ',type(seq2))
        for i in recursive(seq2):
            print('hi')

Input:

recursive('123')

Output:

seq2=  23
Type seq2 =  <class 'str'>
seq2=  3
Type seq2 =  <class 'str'>
seq2=  
Type seq2 =  <class 'str'>
hi
Traceback (most recent call last):
    File "<pyshell#12>", line 1, in <module>
        recursive('123')
    File "C:/Python34/myreqursive(del).py", line 8, in recursive
        for i in recursive(seq2):
    File "C:/Python34/myreqursive(del).py", line 8, in recursive
        for i in recursive(seq2):
TypeError: 'NoneType' object is not iterable

Answer 1

Your else branch doesn't return anything, so the default return value None is given instead.

Rather than just print, at least return the recursive result:

def recursive(seq):
    if not seq:
        return [seq]
    else:
        seq2=seq[1:]
        print('seq2= ',seq2)
        print('Type seq2 = ',type(seq2))
        recursive_result = recursive(seq2)
        for i in recursive_result:
            print('hi')
        return recursive_result