Convert a LinkedList to a String in Python

The challenge

Preloaded for you is a class, struct, or derived data type Node (depending on the language) used to construct linked lists in this challenge:

class Node():
    def __init__(self, data, next = None):
        self.data = data
        self.next = next

Create a function stringify which accepts an argument list/$list and returns a string representation of the list. The string representation of the list starts with the value of the current Node, specified by its data/$data/Data property, followed by a whitespace character, an arrow, and another whitespace character (" -> "), followed by the rest of the list. The end of the string representation of a list must always end with None. For example, given the following list:

Node(1, Node(2, Node(3)))

… its string representation would be:

"1 -> 2 -> 3 -> None"

And given the following linked list:

Node(0, Node(1, Node(4, Node(9, Node(16)))))

… its string representation would be:

"0 -> 1 -> 4 -> 9 -> 16 -> None"

Note that None itself is also considered a valid linked list. In that case, its string representation would simply be "None" (again, depending on the language).

The solution in Python code

Option 1:

Node.__str__ = lambda self: "%s -> %s" % (self.data, self.next)
stringify = str

Option 2:

def stringify(ll):
    r = []
    while ll:
        r, ll = r + [str(ll.data)], ll.next
    return ' -> '.join(r + ['None'])

Option 3:

class Node():
    def __init__(self, data, next = None):
        self.data = data
        self.next = next


def stringify(node):
    k = ''
    while True:
        if node == None:
            k += 'None'
            return k
        else:
            k += '{} -> '.format(node.data)
            node = node.next

Test cases to validate our solution

test.describe("stringify()")

test.it("should pass the example tests as shown in the Description")
test.assert_equals(stringify(Node(0, Node(1, Node(2, Node(3))))), '0 -> 1 -> 2 -> 3 -> None')
test.assert_equals(stringify(None), 'None')
test.assert_equals(stringify(Node(0, Node(1, Node(4, Node(9, Node(16)))))), '0 -> 1 -> 4 -> 9 -> 16 -> None')