metaclass conflict with newstr

Description

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

Contributor guide

Tech stack

python

Domain

backend

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

4

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

half day

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

stale

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

Understanding of Python metaclassesFamiliarity with python future library

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

10

Research direction

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`.