如何使用mypy和@property分配值？

Question

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我正在使用@property装饰器，并为该属性分配了一个值。但是，mypy对我的代码抛出错误（有效）。我做错什么了吗？如何修复我的代码，使mypy不会抱怨？

MVCE

# Core Library
from abc import ABCMeta, abstractmethod


class ListNode(metaclass=ABCMeta):
    def __init__(self, value):
        """Every list node should have a value at initialization."""

    @property
    @abstractmethod
    def value(self):
        """Read the value attribute"""

    @abstractmethod
    def next(self):
        """Read the next attribute"""


class SinglyLinkedListNode(ListNode):
    def __init__(self, value):
        self.value = value
        self.next = None  # Optional[SinglyLinkedListNode]

    @property
    def value(self):
        return self._value

    @value.setter
    def value(self, value):
        self._value = value

    @property
    def next(self):
        return self._next

    @next.setter
    def next(self, next_):
        self._next = next_


def reverse(list_node: ListNode) -> ListNode:
    """Reverse a list."""
    current = list_node
    previous = None

    while current is not None:
        previous, current.next, current = current, previous, current.next  # This throws the error

    return previous

错误：

mpu/datastructures/lists.py:47: error: Cannot assign to a method
mpu/datastructures/lists.py:47: error: Incompatible types in assignment (expression has type "ListNode", variable has type "Callable[[], Any]")
mpu/datastructures/lists.py:47: error: Incompatible types in assignment (expression has type "Callable[[], Any]", variable has type "ListNode")

Answer 1

在您的示例中，可能是错字，而不是

self.next = None

应该是

self._next = None

因为我们用您的代码覆盖property的字段，似乎可以正常工作，但可能不希望这样做。

而且似乎ListNode.next也应该是property

class ListNode(metaclass=ABCMeta):
    ...

    @property
    @abstractmethod
    def next(self):
        """Read the next attribute"""
最后为ListNode.next添加setter

    @next.setter
    def next(self, next_):
        """Write the next attribute"""
使错误消失。

---

BTW用ListNode.next装饰abstractmethod设置程序会导致错误

> mypy test.py
test.py:14: error: Overloaded method has both abstract and non-abstract variants
test.py:19: error: Decorated property not supported
test.py:53: error: Property "next" defined in "ListNode" is read-only
并且似乎是多余的，因为删除SinglyLinkedListNode.next的二传手会导致

> mypy test.py
test.py:37: error: Read-only property cannot override read-write property
同样要使type comment正常工作，我们需要添加type:前缀

class SinglyLinkedListNode(ListNode):
    def __init__(self, value):
        self.value = value
        self._next = None  # type: Optional[SinglyLinkedListNode]
（（我们也可以添加from typing import Optional，因为PyCharm对“ Unresolved reference'Optional'

不满意，但mypy似乎没问题）。