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

With so many different projects impacted by #191, often its best to revert the commit in question and cut a point release to mitigate the issue. Reverts are easier to get in. Later, you can write and discuss a new implementation with the maintainers and if there really is a breaking change, draft a changelog note about it.

I have a fix for the __metaclass__ issue at #211.

I have the same issue with some Google libraries:

  File "/Users/florenthemmi/dev/env/lib/python2.7/site-packages/apache_beam/internal/gcp/json_value.py", line 23, in <module>
    from apitools.base.py import extra_types
  File "/Users/florenthemmi/dev/env/lib/python2.7/site-packages/apitools/base/py/__init__.py", line 21, in <module>
    from apitools.base.py.base_api import *
  File "/Users/florenthemmi/dev/env/lib/python2.7/site-packages/apitools/base/py/base_api.py", line 31, in <module>
    from apitools.base.protorpclite import message_types
  File "/Users/florenthemmi/dev/env/lib/python2.7/site-packages/apitools/base/protorpclite/message_types.py", line 25, in <module>
    from apitools.base.protorpclite import messages
  File "/Users/florenthemmi/dev/env/lib/python2.7/site-packages/apitools/base/protorpclite/messages.py", line 1165, in <module>
    class Field(six.with_metaclass(_FieldMeta, object)):
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

Looking at the 3rd party package code that triggers the error, it boils down to this:

import six


class Meta(type):
    pass


class A(six.with_metaclass(Meta)):
    __metaclass__ = Meta

Removing the __metaclass__ prevents the TypeError.

Got the same issue using a Django package.

File "/usr/local/lib/python2.7/site-packages/autocomplete_light/shortcuts.py", line 8, in <module>
   from .forms import *
 File "/usr/local/lib/python2.7/site-packages/autocomplete_light/forms.py", line 438, in <module>
   class ModelForm(six.with_metaclass(*bases)):
 File "/usr/local/lib/python2.7/site-packages/autocomplete_light/forms.py", line 283, in __new__
   attrs)
 File "/usr/local/lib/python2.7/site-packages/django/forms/models.py", line 208, in __new__
   new_class = super(ModelFormMetaclass, mcs).__new__(mcs, name, bases, attrs)
 File "/usr/local/lib/python2.7/site-packages/django/forms/forms.py", line 41, in __new__
   new_class = super(DeclarativeFieldsMetaclass, mcs).__new__(mcs, name, bases, attrs)
 File "/usr/local/lib/python2.7/site-packages/django/forms/widgets.py", line 153, in __new__
   new_class = super(MediaDefiningClass, mcs).__new__(mcs, name, bases, attrs)
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

class PywbemLoggers(six.with_metaclass(MetaPywbemLoggers)):
    __metaclass__ = MetaPywbemLoggers

As discussed above, this is wrong: you should not combine six.with_metaclass and __metaclass__. However, as many people seem to do this, it’s becoming clear that a fix is needed.

The issue in google-apitools is the same:

# TODO(rafek): Prevent additional field subclasses.
class Field(six.with_metaclass(_FieldMeta, object)):
    """Definition for message field."""

    __initialized = False  # pylint:disable=invalid-name
    __variant_to_type = {}  # pylint:disable=invalid-name

    # TODO(craigcitro): Remove this alias.
    #
    # We add an alias here for backwards compatibility; note that in
    # python3, this attribute will silently be ignored.
    __metaclass__ = _FieldMeta