When a value can be absent, represent that absence explicitly and consistently.

Do

• Use Optional[...]/T | None in type hints for nullable inputs, outputs, and pointers.
• Initialize missing references (e.g., children, root) to None—never to a placeholder like self.
• For empty inputs, set the owning state to None (or return early) rather than creating dummy nodes that look valid.
• Keep function return contracts consistent: if an API can’t produce a result, return None (or use a single sentinel), and type it accordingly.
• If None is only used to mean “use default behavior,” prefer a non-None default argument.

Example (segment-tree style):

from dataclasses import dataclass
from typing import Optional

@dataclass
class Node:
    start: int
    end: int
    value: int = 0
    left: Optional["Node"] = None
    right: Optional["Node"] = None

class SegmentTree:
    def __init__(self, nums: list[int]) -> None:
        self.root: Optional[Node] = None
        if not nums:
            return
        self.root = self.build(0, len(nums) - 1, nums)

Applying this standard prevents null-reference bugs, removes confusing placeholder behavior, and makes absence handling readable and enforceable by both humans and type checkers.