python - 在 Python 中创建参数化类型，但将所有实例设为 "super-type"的子实例

Question

假设我有一个具有 t 属性的 Python 类型。我想创建一个“参数化元类型”，以便执行以下操作:

class MySuperClass(type):
    pass

class MySubClass(MySuperClass):
    # Here is the problem -- How do I define types that contain stuff,
    # independent of an object?
    def __init__(self, t): # Or __getitem__
        self.t = t

    def __instancecheck__(self, instance):
        return isinstance(instance, MySubClass) and instance.t == self.t

    def __subclasscheck__(self, subclass):
        return MySubClass in subclass.__mro__ and subclass.t == self.t

class MyObject(metaclass=MySubClass):
    def __init__(self, t):
        self.t = t

# Test code:
## Both of these, square brackets work too
assert isinstance(MyObject(0), MySubClass(0))
assert not isinstance(MyObject(0), MySubClass(1))

## Ideally
assert isinstance(MyObject(0), MySuperClass) or isinstance(MyObject(0), MySubClass)

目前我收到以下错误:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-99ad08881526> in <module>
     14         return MySubClass in subclass.__mro__ and subclass.t == self.t
     15 
---> 16 class MyObject(metaclass=MySubClass):
     17     def __init__(self, t):
     18         self.t = t

TypeError: __init__() takes 2 positional arguments but 4 were given

Answer 1

可以满足第一部分或要求。但它需要一个辅助检查器类。 MySubClass 是类型的后代，MySubClass(0) 应该是一个类。它足以在 MySubClass 中创建一个内部类 InstanceChecker 类，并将 __instancecheck__ 覆盖它们。

代码可以是:

class MySubClass(MySuperClass):
    def __new__(cls, name, bases=None, namespace=None, *args, **kwargs):
        if bases is not None:
            return super().__new__(cls, name, bases, namespace, **kwargs)
        return cls.InstanceChecker(name)

    class InstanceChecker:
        def __init__(self, t):
            self.t = t
        def __instancecheck__(self, instance):
            return isinstance(instance.__class__, MySubClass) and instance.t == self.t            

class MyObject(metaclass=MySubClass):
    def __init__(self, t):
        self.t = t

# Test code:
## Both of these, square brackets work too
assert isinstance(MyObject(0), MySubClass(0))
assert not isinstance(MyObject(0), MySubClass(1))

顺便说一句，我已经删除了 __subclasscheck__ 覆盖，因为 t 仅在 MyObject 的 instance 属性中/p>

---

或者，元类可以在bases 参数中自动添加父类(super class)。在下面的代码中，MySuperClass 不再是 MySubClass 的父类(super class)，而是 MyObject 的父类(super class):

class MySuperClass():
    pass


class MySubClass(type):
    def __new__(cls, name, bases=None, namespace=None, *args, **kwargs):
        if bases is not None:
            return super().__new__(cls, name, bases + (MySuperClass,), namespace, **kwargs)
        return cls.InstanceChecker(name)
    class InstanceChecker:
        def __init__(self, t):
            self.t = t
        def __instancecheck__(self, instance):
            return isinstance(instance.__class__, MySubClass) and instance.t == self.t

class MyObject(metaclass=MySubClass):
    def __init__(self, t):
        self.t = t

# Test code:
## Both of these, square brackets work too
assert isinstance(MyObject(0), MySubClass(0))
assert not isinstance(MyObject(0), MySubClass(1))

## Ideally
assert isinstance(MyObject(0), MySuperClass)