解决冲突元类(Resolving metaclass conflicts)

I need to create a class that uses a different base class depending on some condition. With some classes I get the infamous:

TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

One example is sqlite3, here is a short example you can even use in the interpreter:

>>> import sqlite3
>>> x = type('x', (sqlite3,), {})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

Answer 1:

除了使用由JDI提到receipe的，你可以直接使用：

class M_C(M_A, M_B):
    pass

class C(A, B):
    __metaclass__ = M_C

Answer 2:

您例如使用sqlite3是无效的，因为它是一个模块，而不是一类。 我也遇到了这个问题。

您的继承人问题：基类有一个元类是不是同一类型的子类。 这就是为什么你会得到一个TypeError 。

我用的变体使用noconflict.py这ActiveState的片段 。 该段需要返工，因为它不是蟒蛇3.X兼容。 无论如何，它应该给你一个总体思路。

问题摘录

class M_A(type):
    pass
class M_B(type):
    pass
class A(object):
    __metaclass__=M_A
class B(object):
    __metaclass__=M_B
class C(A,B):
    pass

#Traceback (most recent call last):
#  File "<stdin>", line 1, in ?
#TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass #of the metaclasses of all its bases

解决方案片断

from noconflict import classmaker
class C(A,B):
    __metaclass__=classmaker()

print C
#<class 'C'>

该代码处方正确解析元类为您服务。

Answer 3:

使用由@迈克尔描述的图案，但具有既Python 2和3的相容性（使用six库）：

from six import with_metaclass

class M_C(M_A, M_B):
    pass

class C(with_metaclass(M_C, A, B)):
    # implement your class here

Answer 4:

据我从以前的答案，明白了才想起我们通常必须手工做的是：

class M_A(type): pass
class M_B(type): pass
class A(metaclass=M_A): pass
class B(metaclass=M_B): pass

class M_C(M_A, M_B): pass
class C:(A, B, metaclass=M_C): pass

但是，我们可以在最后两行现在自动化：

def metaclass_resolver(*classes):
    metaclass = tuple(set(type(cls) for cls in classes))
    metaclass = metaclass[0] if len(metaclass)==1 \
                else type("_".join(mcls.__name__ for mcls in metaclass), metaclass, {})   # class M_C
    return metaclass("_".join(cls.__name__ for cls in classes), classes, {})              # class C

class C(metaclass_resolver(A, B)): pass

由于我们不使用任何特定版本的元类语法这metaclass_resolver对Python 2以及Python 3中。

Answer 5:

当您试图从一个函数，而不是一个类继承也会发生这种情况。

例如。

def function():
    pass

class MyClass(function):
    pass

文章来源: Resolving metaclass conflicts