[英]Finding __metaclass__ used before redefinition in Python
我想重新定義一個__metaclass__但是我想回到如果我沒有重新定義就會使用的元類。
class ComponentMetaClass(type):
def __new__(cls, name, bases, dct):
return <insert_prev_here>.__new__(cls, name, bases, dct)
class Component(OtherObjects):
__metaclass__ = ComponentMetaClass
根據我的理解,默認情況下使用的__metaclass__會經歷檢查類范圍內定義的過程,然后是基礎,然后是全局。 通常你會在重定義中使用type,而且通常是全局類型,但是,我的OtherObjects可能已經重新定義了__metaclass__。 所以在使用類型時,我會忽略它們的定義,它們不會運行,對吧?
編輯:請注意,直到運行時我才知道OtherObjects是什么
正如@unutbu所說:“在一個類層次結構中,元類必須是彼此的子類。也就是說,Component的元類必須是OtherObjects的元類的子類。”
這意味着你的問題比你首先要復雜一點 - 不僅你必須從基類調用適當的元類,但你當前的元類也必須從那時起正確地繼承。
(破解一些代碼,面對奇怪的行為,90分鍾后回來)確實很棘手 - 我必須創建一個接收所需元類作為參數的類,並且__call__方法動態生成一個新的元類,修改其基數並添加它的__superclass屬性。
但是這應該做你想要的更多 - 你只需從BaseComponableMeta
繼承你的所有元類,並通過元類“__superclass”屬性調用層次結構中的超類:
from itertools import chain
class Meta1(type):
def __new__(metacls, name, bases, dct):
print name
return type.__new__(metacls, name, bases, dct)
class BaseComponableMeta(type):
def __new__(metacls, *args, **kw):
return metacls.__superclass.__new__(metacls, *args, **kw)
class ComponentMeta(object):
def __init__(self, metaclass):
self.metaclass = metaclass
def __call__(self, name, bases,dct):
#retrieves the deepest previous metaclass in the object hierarchy
bases_list = sorted ((cls for cls in chain(*(base.mro() for base in bases)))
, key=lambda s: len(type.mro(s.__class__)))
previous_metaclass = bases_list[-1].__class__
# Adds the "__superclass" attribute to the metaclass, so that it can call
# its bases:
metaclass_dict = dict(self.metaclass.__dict__).copy()
new_metaclass_name = self.metaclass.__name__
metaclass_dict["_%s__superclass" % new_metaclass_name] = previous_metaclass
#dynamicaly generates a new metaclass for this class:
new_metaclass = type(new_metaclass_name, (previous_metaclass, ), metaclass_dict)
return new_metaclass(name, bases, dct)
# From here on, example usage:
class ComponableMeta(BaseComponableMeta):
pass
class NewComponableMeta_1(BaseComponableMeta):
def __new__(metacls, *args):
print "Overriding the previous metaclass part 1"
return metacls.__superclass.__new__(metacls, *args)
class NewComponableMeta_2(BaseComponableMeta):
def __new__(metacls, *args):
print "Overriding the previous metaclass part 2"
return metacls.__superclass.__new__(metacls, *args)
class A(object):
__metaclass__ = Meta1
class B(A):
__metaclass__ = ComponentMeta(ComponableMeta)
# trying multiple inheritance, and subclassing the metaclass once:
class C(B, A):
__metaclass__ = ComponentMeta(NewComponableMeta_1)
# Adding a third metaclass to the chain:
class D(C):
__metaclass__ = ComponentMeta(NewComponableMeta_2)
# class with a "do nothing" metaclass, which calls its bases metaclasses:
class E(D):
__metaclass__ = ComponentMeta(ComponableMeta)
在一個類層次結構中,元類必須是彼此的子類。 也就是說, Component
的元類必須是OtherObjects
的元類的子類。
如果你不命名__metaclass__
的Component
,然后元類OtherObjects
將默認使用。
如果ComponentMetaClass
和OtherObjectsMeta
都從type
繼承(獨立):
class OtherObjectsMeta(type): pass
class ComponentMetaClass(type): pass
class OtherObjects(object):
__metaclass__ = OtherObjectsMeta
class Component(OtherObjects):
__metaclass__ = ComponentMetaClass
然后你得到這個錯誤:
TypeError: Error when calling the metaclass bases
metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases
但是如果你將ComponentMetaClass
作為OtherObjectsMeta
子類
class ComponentMetaClass(OtherObjectsMeta): pass
然后錯誤就消失了。
也許我誤解了你的問題。 如果想要ComponentMetaClass.__new__
調用OtherObjectsMeta.__new__
,那么使用super
:
class OtherObjectsMeta(type):
def __new__(meta, name, bases, dct):
print('OtherObjectsMeta')
return super(OtherObjectsMeta,meta).__new__(meta,name,bases,dct)
class ComponentMetaClass(OtherObjectsMeta):
def __new__(meta, name, bases, dct):
print('ComponentMetaClass')
return super(ComponentMetaClass,meta).__new__(meta,name,bases,dct)
關於使用元類的替代方法,在評論中提到。 使用super
:
class Base(object):
def method(self): pass
class Base1(Base):
def method(self):
print('Base1')
super(Base1,self).method()
class Base2(Base):
def method(self):
print('Base2')
super(Base2,self).method()
class Component(Base1,Base2):
pass
c = Component()
c.method()
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