from __future__ import annotations
A rough mental model:
- Without it: annotations are evaluated immediately at definition time
- With it: annotations are stored as strings and resolved later
The Problem
Without from __future__ import annotations, forward references fail because the name isn’t defined yet:
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class Node:
def __init__(self, next: Node | None = None): # NameError: 'Node' is not defined
self.next = next
This fails in older Python versions because Node is used before the class is fully defined.
The Fix
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from __future__ import annotations
class Node:
def __init__(self, next: Node | None = None):
self.next = next
With this import, Python stores annotations as deferred strings ("Node | None") instead of evaluating them immediately at function/class definition time. Node | None is kept as annotation data first, not resolved immediately, so the forward reference works.
