Python の前方参照

PEP 484 -- Type Hints | Python.org

class Tree:
    def __init__(self, left: 'Tree', right: 'Tree'):
        self.left = left
        self.right = right

うーん・・・。

PEP 563 -- Postponed Evaluation of Annotations | Python.org

from __future__ import annotations

まぁ・・・。