Recursive protocols

Protocols can be recursive (self-referential) and mutuallyrecursive. This is useful for declaring abstract recursive collectionssuch as trees and linked lists:

from typing import TypeVar, Optional
from typing_extensions import Protocol
 
class TreeLike(Protocol):
    value: int
 
    @property
    def left(self) -> Optional['TreeLike']: ...
 
    @property
    def right(self) -> Optional['TreeLike']: ...
 
class SimpleTree:
    def __init__(self, value: int) -> None:
        self.value = value
        self.left: Optional['SimpleTree'] = None
        self.right: Optional['SimpleTree'] = None
 
root: TreeLike = SimpleTree(0)  # OK