the perils of returning unicode from __str__ in Python 2 compatible code

dunder_str_unicode.py

# -*- coding: utf-8 -*-
from django.utils.encoding import force_str


class BadStr(object):
    def __init__(self, content):
        self.content = content

    def __str__(self):
        assert isinstance(self.content, unicode), "returning it from __str__ anyway"
        return self.content


class GoodStr(object):
    def __init__(self, content):
        self.content = content

    def __str__(self):
        return force_str(self.content)


def test_case():
    bad = BadStr(u"an object of ClassName")
    good_1 = GoodStr(u"\N{HOT BEVERAGE}")
    good_2 = GoodStr(u"\N{SNOWMAN}")

    template = b"%s and %s"

    this_works = template % (good_1, good_2)

    this_doesnt = template % (bad, good_1)


if __name__ == '__main__':
    test_case()

output.txt

$ python2 dunder_str_unicode.py
Traceback (most recent call last):
  File "dunder_str_unicode.py", line 35, in <module>
    test_case()
  File "dunder_str_unicode.py", line 31, in test_case
    this_doesnt = template % (bad, good_1)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 0: ordinal not in range(128)

Process finished with exit code 1