Inconsistent behavior with slice assignment?

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-12-12.11:58:23.353>
labels = ['interpreter-core', 'type-bug', 'docs']
title = 'Inconsistent behavior with slice assignment?'
updated_at = <Date 2018-05-10.05:16:28.716>
user = 'https://bugs.python.org/MassimilianoCulpo'

bugs.python.org fields:

activity = <Date 2018-05-10.05:16:28.716>
actor = 'Massimiliano Culpo'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Interpreter Core']
creation = <Date 2017-12-12.11:58:23.353>
creator = 'Massimiliano Culpo'
dependencies = []
files = []
hgrepos = []
issue_num = 32288
keywords = []
message_count = 10.0
messages = ['308120', '308123', '308124', '308126', '308129', '308130', '308131', '308133', '316319', '316347']
nosy_count = 6.0
nosy_names = ['steven.daprano', 'r.david.murray', 'skrah', 'docs@python', 'serhiy.storchaka', 'Massimiliano Culpo']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32288'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.5', 'Python 3.6']