In Python, from __future__ import annotations is a special import that changes how type annotations are handled by the interpreter.

📌 What it does

Normally, when you annotate a function or class in Python, the interpreter evaluates the annotations immediately.
That means you can only reference types that are already defined.

But when you use:

from __future__ import annotations

👉 All annotations are stored as strings (instead of being evaluated immediately).
This is called "Postponed Evaluation of Annotations", introduced in PEP 563.

✅ Why is this useful?

1. Forward References (Self-referencing classes)
   Without it, you'd need to wrap type names in quotes if they refer to something not yet defined.

   # Without future import
   class Node:
       def __init__(self, next: 'Node' = None):  # must quote 'Node'
           self.next = next
   

   With from __future__ import annotations:

   from __future__ import annotations
   
   class Node:
       def __init__(self, next: Node | None = None):  # no quotes needed
           self.next = next
   

2. Avoid circular import issues
   Since annotations are stored as strings, Python doesn’t need to resolve imports immediately.
   This prevents many “ImportError: cannot import name …” problems in large projects.

3. Performance optimization
   Storing annotations as strings delays evaluation until needed (e.g., by type checkers or typing.get_type_hints()), which makes function/class definitions faster.

⚡ Example

from __future__ import annotations

def square(x: int) -> int:
    return x * x

class Tree:
    def __init__(self, left: Tree | None = None, right: Tree | None = None):
        self.left = left
        self.right = right

• Without the import, you’d get NameError: name 'Tree' is not defined.

• With the import, Python stores "Tree | None" as a string, and everything works.

🔮 Python 3.11+

In Python 3.11+, this behavior (annotations as strings) is default (PEP 563 was adopted).
So, you usually don’t need to write from __future__ import annotations anymore unless you're working with Python <3.11.

👉 In short:
from __future__ import annotations =
💡 “Don’t evaluate type hints right now, just save them as strings for later.”