[ Python 常見問題 ] How do I type hint a method with the type of the enclosing class?

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Question
I have the following code in python 3:

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1. class Position:  
2.   
3.     def __init__(self, x: int, y: int):  
4.         self.x = x  
5.         self.y = y  
6.   
7.     def __add__(self, other: Position) -> Position:  
8.         return Position(self.x + other.x, self.y + other.y)  

But the return type hinting def __add__(self, other: Position) -> Position will fail.

HowTo
If you are using Python 3.10 or later, it just works. As of today (2019), in 3.7+ you must turn this feature on using a future statement (from __future__ import annotations). In Python 3.6 or below, use a string.

I guess you got this exception:

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1. NameError: name 'Position' is not defined  

This is because Position must be defined before you can use it in an annotation unless you are using Python 3.10 or later.

Python 3.7+: from __future__ import annotations
Python 3.7 introduces PEP 563: postponed evaluation of annotations. A module that uses the future statement from __future__ import annotations will store annotations as strings automatically:

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1. from __future__ import annotations  
2.   
3. class Position:  
4.     def __add__(self, other: Position) -> Position:  
5.         ...  

This is scheduled to become the default in Python 3.10. Since Python still is a dynamically typed language so no type checking is done at runtime, typing annotations should have no performance impact, right? Wrong! Before python 3.7 the typing module used to be one of the slowest python modules in core so if you import typing you will see up to 7 times increase in performance when you upgrade to 3.7.

Python <3.7: use a string
According to PEP 484, you should use a string instead of the class itself:

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1. class Position:  
2.     ...  
3.     def __add__(self, other: 'Position') -> 'Position':  
4.        ...