unicode error fix

test_unicodeerror.py

# -*- coding: utf-8 -*-
from __future__ import unicode_literals
import mock

post = mock.Mock()

def get_side_effect(value, *args):
    if value == 'author':
         return u'me'
    if value == 'tags':
        return [u'£2', u'b']

post.get.side_effect = get_side_effect
get_post = mock.Mock(return_value=post)


def generate_tag_string(post_id, tags=None, new=False):
    """
    Given a post_id, retrieve the tags and combine them
    with the argument `tags`, if any, to create a string
    that joins tags into a command separated string along
    with the author.  If the boolean argument `new` is
    `True`, then add the 'new' tag.
    """
    tags = tags or []
    if new:
        tags.append('new')

    post = get_post(post_id)

    return '{}: {}'.format(post.get('author'), ', '.join(post.get('tags', []) + tags))

def test_tag_unicode_error():
    response = generate_tag_string(1)

    assert response == 'me: £2, b'

def test_tag_default():
    response = generate_tag_string(1, new=True)
    assert response == 'me: £2, b, new'
    response = generate_tag_string(1)
    assert response == 'me: £2, b'

This reproduces the error when executed without the __future__ import.
This only happens on python 2
tags=[] is not a safe default value, as this stores the last call value.
replace with tags=None and in the function body something like tags = tags or []