[Python] Inserting Into Linked List Python

I've create a pretty standard linked list in python with a Node class and LinkedList class. I've also added in methods for LinkedList as follows:

1. add(newNode): Adds an element to the linked list
2. addBefore(valueToFind, newNode): Adds a new node before an element with the value specified.
3. printClean: Prints the linked list

I'm trying to use the addBefore method to perform an insertion, however it will not work if the insertion isn't on the head. I'm not sure why.

class Node:
def __init__(self, dataval =None):
self.dataval = dataval
self.nextval = None

class LinkedList:
def __init__(self, headval =None):
self.headval = headval

def add(self, newNode):
# The linked list is empty
if(self.headval is None):
self.headval = newNode
else:
# Add to the end of the linked list
currentNode = self.headval
while currentNode is not None:
# Found the last element
if(currentNode.nextval is None):
currentNode.nextval = newNode
break
else:
currentNode = currentNode.nextval


def addBefore(self, valueToFind, newNode):
currentNode = self.headval
previousNode = None
while currentNode is not None:
# We found the element we will insert before
if (currentNode.dataval == valueToFind):
# Set our new node's next value to the current element
newNode.nextval = currentNode

# If we are inserting at the head position
if (previousNode is None):
self.headval = newNode
else:
# Change previous node's next to our new node
previousNode.nexval = newNode
return 0

# Update loop variables
previousNode = currentNode
currentNode = currentNode.nextval
return -1

def printClean(self):
currentNode = self.headval
while currentNode is not None:
print(currentNode.dataval, end='')
if(currentNode.nextval != None):
print("->", end='')
currentNode = currentNode.nextval
else:
return

testLinkedList = LinkedList()
testLinkedList.add(Node("Monday"))
testLinkedList.add(Node("Wednesday"))
testLinkedList.addBefore("Wednesday", Node("Tuesday"))
testLinkedList.printClean()

Monday->Wednesday
​

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