`_v` can be read and written from the outside of the class so the generic class should be invariant properly.

*Memo:

• mypy --strict test.py
• mypy 1.19.1
• Python 3.14.0
• Windows 11

v can be read and written from the outside of the class so the generic class is invariant properly as shown below:

class Cls[T]:
    v: T = cast(T, 100) # Invariant

cls: Cls[int] = Cls[int]()

# It indicates `Invariant`.
cls1: Cls[float] = cls # Errpr
cls2: Cls[int] = cls   # No error
cls3: Cls[bool] = cls  # Error

# Read & Write
print(cls.v) # No error
cls.v = 200  # No error

Now, _v can be read and written from the outside of the class but the generic class is covariant improperly as shown below so it should be invariant properly:

*Memo:

• A single leading underscore(_abc) just indicates Internal use so it's not private(Readable & Writable).

class Cls[T]:
    _v: T = cast(T, 100) # Covariant

cls: Cls[int] = Cls[int]()

# It indicates `Covariant`.
cls1: Cls[float] = cls # No error
cls2: Cls[int] = cls   # No error
cls3: Cls[bool] = cls  # Error

# Read & Write
print(cls._v) # No error
cls._v = 200  # No error

首先，在所述的 mypy 1.19.1 和 Python 3.14.0 版本下，使用 mypy --strict 复现 test.py 中的报告。追踪 mypy 如何为具有公共属性和单下划线属性的泛型类推断方差。完成的标准是 _v 示例被视为不变，同时保留所示的有效和无效赋值。