Самосознание аннотаций типа в Python

Question

Я пытаюсь выяснить, как работает саморепликация типов с python3's type annotations - в документах ничего не говорится об этом.

В качестве примера:

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 

Этот код генерирует ошибку:

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 

Это с помощью Python 3.5.1

Answer 1

PEP 0484 - Type Hints - The problem of forward declarations адреса вопрос:

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]}