类型化python:在类定义中使用类自己的类型 [英] typed python: using the classes' own type inside class definition

问题描述

以下代码未按预期工作.显然，我不能在类定义中使用类自己的类型:

The following code does not work as expected. Apparently, I cannot use the classes' own type inside class definition:

class Foo:
    def __init__(self, key :str) -> None:
        self.key = key

    def __eq__(self, other :Foo) -> bool:
        return self.key == other.key

print('should be true: ', Foo('abc') == Foo('abc'))
print('should be false: ', Foo('abc') == Foo('def'))

运行结果为:

Traceback (most recent call last):
  File "class_own_type.py", line 1, in <module>
    class Foo:
  File "class_own_type.py", line 5, in Foo
    def __eq__(self, other :Foo) -> bool:
NameError: name 'Foo' is not defined

此外，使用 mypy 检查代码返回:

Also, checking the code with mypy returns:

class_own_type.py:5: error: Argument 1 of "__eq__" incompatible with supertype "object"

如何更正此代码以使其对 Python 和 mypy 均有效?

How can I correct this code to be valid, both for Python and for mypy?

推荐答案

名称 Foo 尚未绑定，因为在您尝试使用时尚未定义该类本身名称(请记住:函数参数在函数定义时计算，而不是在函数调用时).

The name Foo isn't bound yet, because the class itself has not yet been defined at the time that you try to use the name (remember: function arguments are evaluated at function definition time, not at function call time).

从 Python 3.7+ 开始，您可以通过添加以下内容推迟对注释的评估在模块顶部导入:

From Python 3.7+ you can postpone evaluation of annotations by adding this import at the top of the module:

from __future__ import annotations

在 Python 3.10 中，延迟评估将成为默认行为.对于 Python <3.7、可以使用字符串字面量来延迟类型的求值:

In Python 3.10 postponed evaluation will become the default behavior. For Python < 3.7, you can use string literals to delay evaluation of the type:

class Foo:
    def __init__(self, key :str) -> None:
        self.key = key

    def __eq__(self, other: 'Foo') -> bool:
        return self.key == other.key

print('should be true: ', Foo('abc') == Foo('abc'))
print('should be false: ', Foo('abc') == Foo('def'))

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