TypeError: super(type, obj): obj must be an instance or subtype of type. Error when calling super after metaclass creation

Question

假设我使用了一个库，其源代码是按照这种形式编写的：

class SuperLibraryDemo:
    def __init__(self, test) -> None:
        self.test = test

    def demo(self) -> str:
        return "Returning from demo method with name: %s" % self.test


class LibraryDemo(SuperLibraryDemo):
    def __init__(self, test: str = "something") -> None:
        super(LibraryDemo, self).__init__(test)
        print("At LibraryDemo __init__ method: This should have been skipped")

    def demo(self) -> None:
        super().demo()

记住那是图书馆。我不应该根据需要调整它的源代码。但是，出于超出此问题范围的原因，我需要切换 __init__ 方法在 LibraryDemo 中调用的内部代码。

考虑到这个目标，我决定在元类的帮助下编写 CustomLibraryDemo，如下所示：

class MetaDemo(type):
    def __new__(mcs, class_name: str, bases: Tuple[Type, ...], class_dict: Dict[str, Any]):
        basis = bases[0]
        c_attrs = dict(basis.__dict__)
        prior_c_process_bases = basis.__base__
        c_attrs["__init__"] = lambda self, settings: prior_c_process_bases.__init__(self, settings)
        new_bases = types.new_class(basis.__qualname__, basis.__bases__,
                                    exec_body=lambda np: MetaDemo.populate_class_dict(np, c_attrs))
        return super(MetaDemo, mcs).__new__(mcs, class_name, (new_bases,), class_dict)

    @staticmethod
    def populate_class_dict(namespace: Dict[str, Any], attr: Dict[str, Any]) -> None:
        for key, value in attr.items():
            namespace[key] = value

class CustomLibraryDemo(LibraryDemo, metaclass=MetaDemo):
    def __init__(self, test: Optional[str] = None) -> None:
        super(CustomLibraryDemo, self).__init__(test)
        print("At CustomDemo __init__ method: This message should appear")

    def test(self) -> None:
        print("In test method at CustomLibraryDemo class: %s" % self.test)

虽然乍一看这种方法似乎对我有用，但当我调用 CustomLibraryDemo().demo() 时出现错误说：

TypeError: super(type, obj): obj must be an instance or subtype of type

为什么？

Answer 1

您可能不需要自定义元数据class；相反，只需将参数调整为 super.

class CustomLibraryDemo(LibraryDemo):
    def __init__(self, test: Optional[str] = None) -> None:
        super(LibraryDemo, self).__init__(test)
        print("At CustomDemo __init__ method: This message should appear")

    def test(self) -> None:
        print("In test method at CustomLibraryDemo class: %s" % self.test)

使用 LibraryDemo 而不是 CustomerLibraryDemo 会导致 super 在决定接下来使用哪个 class 时进一步开始 MRO。

% python3 tmp.py
At CustomDemo __init__ method: This message should appear

Answer 2

这个解决了我的问题。对于我的情况，在 __new__ 方法签名上为 bases 参数更改 basis.__bases__ 可以解决问题。这样，new_bases 变量的语法就变成了：

new_bases = types.new_class(basis.__qualname__, bases,
                                    exec_body=lambda np: MetaDemo.populate_class_dict(np, c_attrs))

顺便说一下，对于这段代码可以简化为：

new_bases = type(basis.__qualname__, bases, c_attrs)

删除 populate_class_dict 元类方法。

TypeError: super(type, obj): obj must be an instance or subtype of type. Error when calling super after metaclass creation

TypeError: super(type, obj): obj must be an instance or subtype of type. Error when calling super after metaclass creation

python

metaprogramming

python-3.x