I’m trying to make a Link-List Node class for a Link-List, as shown in the code:

class LNode:
    def __init__(self, data=None, pnext=None):
        self.data = data
        self.pnext = pnext

    def __str__(self):
        return f"{self.data} -> {self.pnext}"

    def __repr__(self):
        return f"{self.data} -> {self.pnext}"

    def __len__(self):
        print("len method called...")
        cnt = 1
        return cnt

    def headInsert(self, nextNode):
        nextNode.pnext = self.pnext
        self.pnext = nextNode

    def headInsert_woh(self, nextNode):
        nextNode.pnext = self
        return nextNode

    def tailInsert(self, nextNode):
        print("tail-insert called...")
        tail = self
        while tail.pnext:
            tail = tail.pnext
        tail.pnext = nextNode
        return self

    def __add__(self, other):
        return self.tailInsert(other)

After definition, I tried codes below:

a = LNode(1)
for i in range(2, 6):
    a += LNode(i)
print(a)

Strangely, __len__ method will be called repeatedly and recursively when the tail-insert method is called, or when the node pointer of the Link-List node moves. As shown below:

tail-insert called......
tail-insert called......
len method called......
tail-insert called......
len method called......
len method called......
tail-insert called......
len method called......
len method called......
len method called......
1 -> 2 -> 3 -> 4 -> 5 -> None

But why? I thought __len__ is the implementation of BIF len(), why it will be called here? Thanks a lot.