如何向 mypy 指示对象具有某些属性？

我正在使用一些从父类派生的类（Widget）；在孩子们中，有些人具有某些属性（posx and posy）但有些则不然。

import enum
from dataclasses import dataclass
from typing import List


class Color(enum.IntEnum):
    GLOWING_IN_THE_DARK = enum.auto()
    BROWN_WITH_RAINBOW_DOTS = enum.auto()


@dataclass
class Widget:
    """Generic class for widget"""


@dataclass
class Rectangle(Widget):
    """A Color Rectangle"""

    posx: int
    posy: int
    width: int = 500
    height: int = 200
    color: Color = Color.BROWN_WITH_RAINBOW_DOTS


@dataclass
class Group(Widget):
    children: List[Widget]


@dataclass
class Button(Widget):
    """A clickable button"""

    posx: int
    posy: int
    width: int = 200
    height: int = 100
    label: str = "some label"

即使仅使用具有这些属性的小部件进行一些过滤后，mypy无法认识到他们应该认识到的。

有没有办法表明mypy我们有一个具有给定属性的对象？

例如，以下函数和调用：

def some_function_that_does_something(widgets: List[Widget]):
    """A useful docstring that says what the function does"""
    widgets_with_pos = [w for w in widgets if hasattr(w, "posx") and hasattr(w, "posy")]

    if not widgets_with_pos:
        raise AttributeError(f"No widget with position found among list {widgets}")

    first_widget = widgets_with_pos[0]
    pos_x = first_widget.posx
    pos_y = first_widget.posy
    print(f"Widget {first_widget} with position: {(pos_x, pos_y)}")


some_widgets = [Group([Rectangle(0, 0)]), Button(10, 10, label="A button")]
some_function_that_does_something(some_widgets)

将返回预期的结果：Widget Button(posx=10, posy=10, width=200, height=100, label='A button') with position: (10, 10)

But mypy会抱怨：

__check_pos_and_mypy.py:53: error: "Widget" has no attribute "posx"
        pos_x = first_widget.posx
                ^
__check_pos_and_mypy.py:54: error: "Widget" has no attribute "posy"
        pos_y = first_widget.posy
                ^
Found 2 errors in 1 file (checked 1 source file)

怎么做？

也许，一种方法是改变类的设计：

• 的子类Widget与位置（例如WidgetWithPos)
• Rectangle and Button将从这个类派生
• 我们在函数中指出：widget_with_pos: List[WidgetWithPos] = ...

...但是，我无法改变课程的原始设计mypy可能仍然会抱怨类似的事情：

List comprehension has incompatible type List[Widget]; expected List[WidgetWithPos]

当然，我们可以放一堆# type:ignore但这会使代码变得混乱，我相信有一种更聪明的方法；）

Thanks!

这是 Alex Waygood 的一个小变化answer https://stackoverflow.com/a/69336474/1393162，删除cast。诀窍是把@runtime_checkable https://docs.python.org/3.9/library/typing.html#typing.runtime_checkableProtocol 类上的装饰器。它只是让isinstance() do the hasattr() checks.

import sys
from dataclasses import dataclass
from typing import List

# Protocol has been added in Python 3.8+
# so this makes the code backwards-compatible
# without adding any dependencies
# (typing_extensions is a MyPy dependency already)

if sys.version_info >= (3, 8):
    from typing import Protocol, runtime_checkable
else:
    from typing_extensions import Protocol, runtime_checkable


@dataclass
class Widget:
    """Generic class for widget"""


@runtime_checkable
class WithPos(Protocol):
    """Minimum interface of all widgets that have a position"""
    posx: int
    posy: int


def some_function_that_does_something(widgets: List[Widget]):
    """A useful docstring that says what the function does"""
    widgets_with_pos = [w for w in widgets if isinstance(w, WithPos)]

    if not widgets_with_pos:
        raise AttributeError(f"No widget with position found among list {widgets}")

    first_widget = widgets_with_pos[0]
    pos_x = first_widget.posx
    pos_y = first_widget.posy
    print(f"Widget {first_widget} with position: {(pos_x, pos_y)}")

以下代码（使用原始问题中定义的其他子类）:

w1 = Group([])
w2 = Rectangle(2, 3)
some_function_that_does_something([w1, w2])

进一步阅读

作为参考，以下是 Alex 在其答案中包含的一些链接：

• Python 文档 https://docs.python.org/3/library/typing.html#typing.Protocol for typing.Protocol
• PEP 544 https://www.python.org/dev/peps/pep-0544/，介绍typing.Protocol并解释结构子类型的概念。
• MyPy 文档 https://mypy.readthedocs.io/en/stable/protocols.html for Protocols 和结构子类型。