Python function

class LinkedNode: # DO NOT MODIFY THIS CLASS # __slots__ = '_value', '_next' def __init__(self, value, next=None): """ DO NOT EDIT Initialize a node :param value: value of the node :param next: pointer to the next node in the LinkedList, default is None """ self._value = value # element at the node self._next = next # reference to next node in the LinkedList def __repr__(self): """ DO NOT EDIT String representation of a node :return: string of value """ return str(self._value) __str__ = __repr__ def get_value(self): return self._value def get_next(self): return self._next def set_value(self, value): self._value = value def set_next(self, next): self._next = next # IMPLEMENT THESE FUNCTIONS - DO NOT MODIFY FUNCTION SIGNATURES # def insert(value, node=None): """ insert a new node to the end of the list that has head node :param value: the value that we are inserting :param node: the head node of the list that we are handling :return: the starting node of the linked list """ if node is None: node = LinkedNode(value) return node elif node._next is None: new_node = LinkedNode(value) node.set_next(new_node) return node else: insert(value, node._next) return node def to_string(node, result=""): """ The values should be separated by a comma and a single space, with no leading or trailing comma Return an empty string if there are no nodes in the list. Recommended Time complexity O(n2). :param node: the head node of the list that we are dealing :param result: the string we want to return :return: a string representation of the list """ if node is None: return "" else: if node._next is None: result += str(node._value) print(result) # this will print out the correct result string return result # but here it returns None else: result += str(node._value) result += ", " to_string(node._next, result)

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test case:

import unittest from Recursion import insert, remove, remove_all, to_string, search, sum_list, \ count, reverse, length, list_percentage

class TestProject2(unittest.TestCase):

def test_to_string(self): list1 = insert(0) insert(1, list1) insert(2, list1) insert(3, list1) print(to_string(list1)) #it prints out None here, but in the function it prints out the right string assert to_string(list1) == "0, 1, 2, 3"

if __name__ == "__main__": unittest.main() _______________________________________________

for the function to string, it won't return the result properly. but it does print out the right string in the function.

what is the issue?