[英]TypeError: super(type, obj): obj must be an instance or subtype of type ONLY when my metaclass is imported
[英]TypeError: super(type, obj): obj must be an instance or subtype of type. Error when calling super after metaclass creation
假設我使用了一個庫,他們的源代碼是按照這個形狀編寫的:
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()
請記住,這是一個圖書館。 我不應該根據我的需要調整它的源代碼。 但是,出於此問題的LibraryDemo
之外的原因,我需要在 LibraryDemo 中切換__init__
方法調用的內部代碼。
考慮到這個目標,我決定在元類的幫助下編寫一個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)
雖然乍一看這種方法似乎對我有用,但當我調用CustomLibraryDemo().demo()
時出現錯誤:
TypeError: super(type, obj): obj must be an instance or subtype of type
為什么?
您可能不需要自定義元類; 相反,只需將 arguments 調整為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)
在決定下一步使用哪個 class 時,使用LibraryDemo
而不是CustomerLibraryDemo
會導致super
沿着 MRO 進一步啟動。
% python3 tmp.py
At CustomDemo __init__ method: This message should appear
這個問題解決了我的問題。 就我而言,在bases
方法簽名上__new__
basis.__bases__
參數可以解決問題。 這樣, new_bases
變量的語法變成:
new_bases = types.new_class(basis.__qualname__, bases,
exec_body=lambda np: MetaDemo.populate_class_dict(np, c_attrs))
順便說一句,這段代碼可以簡化為:
new_bases = type(basis.__qualname__, bases, c_attrs)
刪除populate_class_dict
元類方法。
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