metaclass conflict with newstr

描述

from future.builtins import str
from future.utils import with_metaclass

class MetaClass(type):
    pass

class TestClass(with_metaclass(MetaClass, str)):
    pass

Traceback (most recent call last):
  File "foo.py", line 7, in <module>
    class TestClass(with_metaclass(MetaClass, str)):
  File "/usr/local/lib/python2.7/site-packages/future/utils/__init__.py", line 137, in __new__
    return meta(name, bases, d)
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

贡献者指南

技术栈

python

领域

backend

议题类型

bug

难度面向新贡献者的预计实现难度，1 表示很小改动，5 表示专家级工作。

4

预计时间有经验贡献者完成调查、实现、测试并准备 pull request 的粗略时间范围。

half day

活动状态议题当前的可参与程度：新鲜、活跃、陈旧、阻塞或等待维护者输入。

stale

清晰度议题是否清楚说明期望改动、验收标准和下一步。

clear

前置要求

Understanding of Python metaclassesFamiliarity with python future library

新手友好度1-100 的估计分数，表示该议题对首次贡献者的友好程度。

10

研究方向

Investigate the `with metaclass` function in `future/utils/ init .py` (line 137). Understand how it creates a new class and why metaclass conflict occurs when combining with `newstr`. Look at issue #91 for similar context and ensure that the metaclass of the derived class is a subclass of the metaclasses of all its bases. Possibly adjust the metaclass resolution to handle built in types like `str` from `future.builtins`.