Question

I am trying to write merge sort and stuck here.

What is the problem here with my code? I am trying to implement it without referring any resources and unnecessarily writing this line since some dumb rule in Stackoverflow forces to me explain my code.

def merge_sort(A):
    if len(A) <= 1:
        return A

    #split list in 2
    mid = len(A)/2
    B = A[:mid]
    C = A[mid:]

    B = merge_sort(B)
    C = merge_sort(C)

    #merge
    result = []
    while len(B) > 0 and len(C) > 0:
        if B[0] > C[0]:
            result.append(C.pop(0))
        else:
            result.append(B.pop(0))

    if len(B) > 0:
        result.extend(merge_sort(B))
    else:
        result.extend(merge_sort(C))



print merge_sort([8, 2, 1, 1, 4, 45, 9, 3])

I get this error:

Traceback (most recent call last):
  File "merge_sort.py", line 31, in <module>
    print merge_sort([8, 2, 1, 1, 4, 45, 9, 3])
  File "merge_sort.py", line 11, in merge_sort
    B = merge_sort(B)
  File "merge_sort.py", line 16, in merge_sort
    while len(B) > 0 and len(C) > 0:
TypeError: object of type 'NoneType' has no len()

Answer 1

You merge_sort() function needs to

return result 

at the end but it does not. Functions return None by default and this is why you get the error.