`_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

まず、指定されたバージョンの mypy 1.19.1 と Python 3.14.0 で、mypy --strict を使って test.py の報告を再現します。公開属性と単一アンダースコア属性を持つジェネリッククラスについて、mypy がどのように変性を推論するかを追跡します。_v の例が不変として扱われ、示されている有効な代入と無効な代入が維持されれば完了です。