How to clear this AttributeError "nonetype"?

Question

I'm learning datastructure while compiling this mergesort linkedlist . I got this error. I tried so much but didn't work. Can any one tell me what's wrong? please .

Traceback (most recent call last): File "C:\Users**\AppData\Local\Programs\Python\Python37\merge_sort_ver_2.0 linked list.py", line 122, in

     sorted_list=merge_sort(lst)

File "C:\Users**\AppData\Local\Programs\Python\Python37\merge_sort_ver_2.0 linked list.py", line 75, in merge_sort

return merge(left,right)

File "C:\Users**\AppData\Local\Programs\Python\Python37\merge_sort_ver_2.0 linked list.py", line 94, in merge

left_head=left.head

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

def merge_sort(ll):
    if ll.length()==1:
        return ll
    elif ll.head is None:
        return ll
    left_half,right_half=split(ll)
    left=merge_sort(left_half)
    right=merge_sort(right_half)
    return merge(left,right)
def split(ll):
    if ll.head==None or ll==None:
        left_half=ll
        right_half=None
        return left_half,right_half
    else:
        size=ll.length()
        mid=size//2
        mid_node=ll.nodeAt(mid-1)
        right_half=ll
        left_half=LinkedList()
        left_half.head=mid_node.next
        mid_node.next=None
        return left_half,right_half
def merge(left,right):
    merged=LinkedList()
    merged.tailinsert(0)
    current=merged.head
    left_head=left.head
    right_head=right.head
    
    while left_head or right_head:
        if left_head is None:
            current.next=right_head
            right_head=right_head.next
        elif right_head is None:
            current.next=left_head
            left_head=left_head.next
        else:
    merged.head=head
    return merged.printlist()
lst=LinkedList()
lst.tailinsert(14)
lst.tailinsert(46)
lst.tailinsert(43)
lst.tailinsert(27)
sorted_list=merge_sort(lst)
print(sorted_list)

Answer 1

thanks Jérômei for your tips i cleared the error

def merge_sort(ll):

    if ll.length()==1:
        return ll
    elif ll.head is None:
        return ll
        
    
    left_half,right_half=split(ll)
    left=merge_sort(left_half)
    right=merge_sort(right_half)
    return merge(left,right)

def split(ll):

    if ll.head==None or ll==None:
        left_half=ll
        right_half=None
        return left_half,right_half
    else:
        size=ll.length()
        mid=size//2
        mid_node=ll.nodeAt(mid-1)
        right_half=LinkedList()
        left_half=ll
        right_half.head=mid_node.next
        mid_node.next=None
        return left_half,right_half


def merge(left,right):
    merged=LinkedList()
    merged.tailinsert(0)
    current=merged.head
    left_head=left.head
    right_head=right.head
    while left_head and right_head :
        left_data=left_head.data
        right_data=right_head.data
        if left_data < right_data:
            current.next=left_head
            left_head=left_head.next
        elif right_data <left_data:
            current.next=right_head
            right_head=right_head.next
        current=current.next    
            
    while right_head is not  None  :
        current.next=right_head
        right_head=right_head.next
        current=current.next
    while left_head is not None  :
        current.next=left_head
        left_head=left_head.next    
        current=current.next
         
    current=current.next        
    head=merged.head.next
    merged.head=head
    print("merged 1:")        
    merged.printlist()      

    return merged
lst=LinkedList()
lst.tailinsert(14)
lst.tailinsert(46)
lst.tailinsert(43)
lst.tailinsert(27)
merge_sort(lst)

**output:
merged 1:
14
46
merged 1:
27
43
merged 1:
14
27
43
46**

Answer 2

def merge_sort(ll):
    if ll.length()==1:
        return ll
    elif ll.head is None:
        return ll                    # Here, you return None
    left_half,right_half=split(ll)
    left=merge_sort(left_half)       # Then by recursion, here left=None
    right=merge_sort(right_half)
    return merge(left,right)         # Then you inject it in merge

def split(ll):
    if ll.head==None or ll==None:
        left_half=ll
        right_half=None
        return left_half,right_half
    else:
        size=ll.length()
        mid=size//2
        mid_node=ll.nodeAt(mid-1)
        right_half=ll
        left_half=LinkedList()
        left_half.head=mid_node.next
        mid_node.next=None
        return left_half,right_half

def merge(left,right):                # Then here, left is None
    merged=LinkedList()
    merged.tailinsert(0)
    current=merged.head
    left_head=left.head               # Then boom because None.head
    right_head=right.head

Now, for a bit of method.

When facing this kind of error, use print to display the arguments of your functions. This will help you a lot.

def merge(left,right):
    print(f"merge({left},{right})")

Also when posting such question, include the traceback with the exact error. It will help others here help you.