type() argument 1 must be string, not newstr

[Ionaru]

Given a Python 2 codebase that already uses __future__ imports heavily, when dynamically creating classes, the first argument of type() is wrapped in str().

# -*- coding: utf-8 -*-
from __future__ import absolute_import, division, print_function, unicode_literals

Thing = type(str('Thing'), (object, ), {'attribute': 5})

thing = Thing()
print(thing.attribute)  # 5

When running futurize, the str() gets replaced with the newstr from builtins

# -*- coding: utf-8 -*-
from __future__ import absolute_import, division, print_function, unicode_literals

from builtins import str

Thing = type(str('Thing'), (object, ), {'attribute': 5})  # TypeError

thing = Thing()
print(thing.attribute)

However, because type() expects the first argument to be a string type, it throws an error when running on Python 2:

Traceback (most recent call last):
  File "/snip/type_example.py", line 5, in <module>
    Thing = type(str('Thing'), (object, ), {'attribute': 5})
TypeError: type() argument 1 must be string, not newstr

This seems like a bug because it breaks Python 2 compatibility.

Instead, the code should be replaced with:

# -*- coding: utf-8 -*-
from __future__ import absolute_import, division, print_function, unicode_literals

from future.utils import bytes_to_native_str

Thing = type(bytes_to_native_str(b'Thing'), (object, ), {'attribute': 5})

thing = Thing()
print(thing.attribute)

This will make the code compatible with both Python 2 and Python 3.

[jmadler]

Can you submit a bugfix please? I'll include it in a release in the next few days.