add support of 'six.with_metaclass'

[fmigneault]

Steps

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Description

Fixes issue #713 by providing corresponding transform and unit tests.

Type of Changes

	Type
✓	🐛 Bug fix
✓	✨ New feature

Related Issue

Closes #713

[fmigneault]

flagged pylint issues are related to very recent merge of pylint-dev/pylint#3782 with fix of issue pylint-dev/pylint#3468

[tkukushkin]

@fmigneault please pay attention to #713 (comment). This code

from enum import Enum, EnumMeta import six class FooMeta(EnumMeta): pass class Foo(six.with_metaclass(FooMeta, Enum)): bar = 1

produces error with this fix:

foo.py:3: [W0611 unused-import] Unused import six 

[fmigneault]

@tkukushkin
I don't know enough the internals of astroid to address that specific issue of #841 (comment). Maybe @PCManticore (or other dev from astroid) can help?

The problem in is that after six.with_metaclass(FooMeta, Enum) is processed, the ClassDef effectively becomes:

ClassDef.FooMeta(name='FooMeta', doc=None, decorators=None, bases=[<Name.EnumMeta l.6 at 0x7ff0d24e1a10>], body=[<Pass l.7 at 0x7ff0d24e1a90>]) 

Therefore there is no more reference to six as seen in the source code. At runtime, the class definition is "converted" and doesn't need six anymore. Is there some way to maintain that reference somewhere in the ClassDef for the check to find it?

[tkukushkin]

@fmigneault I completely understand the cause of this problem and I don't know not hacky solutions and PCManticore did not respond to my comment. That's why I didn't make pull request with this transform.

[PCManticore]

Overall the change looks good, but we need to drop the inference check from the predicate function as it can result in weird inference bugs.

[PCManticore]

Unfortunately we cannot use inference in predicate functions. Here you would need o unpack the arguments for checking if they refer to six.with_metaclass or not (by unpacking I mean either checking explicitly for Name or Attribute instances that match against six.with_metaclass)

[fmigneault]

Should be good now. Let me know if it is still not what like you meant.

[PCManticore]

Thanks for the PR @fmigneault !