Python 3.5+ 中的递归类型 [英] Recursive Typing in Python 3.5+

问题描述

在 Python 3.5 中，添加了类型注释(请参阅此处).

In Python 3.5, type annotations were added (see here).

有没有办法定义递归类型的注解，比如树状结构?

Is there a way of defining recursive type annotations, such as for a tree-like structure?

class Employee(object):
    def __init__(self, name: str, reports: List[Employee]):
       self.name = name
       self.reports = reports

在上面，注释 List[Employee] 似乎不起作用.运行代码导致此错误:

In the above, it doesn't seem as though the annotation List[Employee] works. Running the code results in this error:

NameError: name 'Employee' 未定义

推荐答案

您可以使用 前向引用

通常发生这种情况的一种情况是定义容器类，其中定义的类出现在签名中的一些方法.例如下面的代码(开始的一个简单的二叉树实现)不起作用:

A situation where this occurs commonly is the definition of a container class, where the class being defined occurs in the signature of some of the methods. For example, the following code (the start of a simple binary tree implementation) does not work:

class Tree:
    def __init__(self, left: Tree, right: Tree):
        self.left = left
        self.right = right

为了解决这个问题，我们写道:

To address this, we write:

class Tree:
    def __init__(self, left: 'Tree', right: 'Tree'):
        self.left = left
        self.right = right

允许使用字符串文字作为类型提示的一部分，例如例子:

It is allowable to use string literals as part of a type hint, for example:

class Tree:
    ...
    def leaves(self) -> List['Tree']:

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