Self-reference or forward-reference of type annotations in Python

Question

I'm trying to figure out how self-reference of types work with python3's type annotations - the docs don't specify anything regarding this.

As an example:

from typing import TypeVar, Optional, Generic

T = TypeVar('T')
class Node(Generic[T]):
    left = None
    right = None
    value = None

    def __init__(
        self, value: Optional[T],
        left: Optional[Node[T]]=None,
        right: Optional[Node[T]]=None,
    ) -> None:
        self.value = value
        self.left = left
        self.right = right

This code generates the error:

Traceback (most recent call last):
  File "node.py", line 4, in <module>
    class Node(Generic[T]):
  File "node.py", line 12, in Node
    right: Optional[Node[T]]=None,
NameError: name 'Node' is not defined

This is using Python 3.5.1

Answer 1

PEP 0484 - Type Hints - The problem of forward declarations addresses the issue:

The problem with type hints is that annotations (per PEP 3107 , and similar to default values) are evaluated at the time a function is defined, and thus any names used in an annotation must be already defined when the function is being defined. A common scenario is a class definition whose methods need to reference the class itself in their annotations. (More general, it can also occur with mutually recursive classes.) This is natural for container types, for example:

...

As written this will not work, because of the peculiarity in Python that class names become defined once the entire body of the class has been executed. Our solution, which isn't particularly elegant, but gets the job done, is to allow using string literals in annotations. Most of the time you won't have to use this though -- most uses of type hints are expected to reference builtin types or types defined in other modules.

from typing import TypeVar, Optional, Generic

T = TypeVar('T')
class Node(Generic[T]):
    left = None
    right = None
    value = None

    def __init__(
        self,
        value: Optional[T],
        left: Optional['Node[T]']=None,
        right: Optional['Node[T]']=None,
    ) -> None:
        self.value = value
        self.left = left
        self.right = right

---

>>> import typing
>>> typing.get_type_hints(Node.__init__)
{'return': None,
 'value': typing.Union[~T, NoneType],
 'left': typing.Union[__main__.Node[~T], NoneType],
 'right': typing.Union[__main__.Node[~T], NoneType]}