TypeError:super（类型，obj）：obj必须是类型的实例或子类型。创建元类后调用super时出错

Question

让我们想象一下，我使用了一个库，其源代码是按照以下形状编写的：

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()

记住那是一个图书馆。我不应该根据自己的需要修改它的源代码。 但是，我需要切换__init__方法在LibraryDemo内调用的内部代码，原因超出了这个问题的范围

考虑到这个目标，我决定在元类的帮助下编写一个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)

虽然这种方法乍一看似乎对我有效，但当我调用CustomLibraryDemo().demo()时，我得到一个错误，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类；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在决定下一个使用哪个类时沿着MRO进一步开始

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