"utf8-sig" missing from codecs (inconsistency)

[PeterLudemann]

BPO	39155
Nosy	@vstinner, @ezio-melotti

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-12-29.18:14:13.531>
labels = ['3.8', 'type-bug', '3.7', 'expert-unicode']
title = '"utf8-sig" missing from codecs (inconsistency)'
updated_at = <Date 2019-12-29.18:14:13.531>
user = 'https://bugs.python.org/PeterLudemann'

bugs.python.org fields:

activity = <Date 2019-12-29.18:14:13.531>
actor = 'Peter Ludemann'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Unicode']
creation = <Date 2019-12-29.18:14:13.531>
creator = 'Peter Ludemann'
dependencies = []
files = []
hgrepos = []
issue_num = 39155
keywords = []
message_count = 1.0
messages = ['358996']
nosy_count = 3.0
nosy_names = ['vstinner', 'ezio.melotti', 'Peter Ludemann']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39155'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

Linked PRs

• gh-83336: Add alias for consistency to utf-8-sig #136530

[PeterLudemann]

In general, 'utf8' and 'utf-8' are interchangeable in the codecs (and in many parts of the Python library). However, 'utf8-sig' is missing ... and it happens to also be generated by lib2to3.tokenize.detect_encoding.

>>> import codecs
>>> codecs.getincrementaldecoder('utf-8-sig')()
<encodings.utf_8_sig.IncrementalDecoder object at 0x7fecbcdbbc10>
>>> codecs.getincrementaldecoder('utf8-sig')()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/codecs.py", line 987, in getincrementaldecoder
    decoder = lookup(encoding).incrementaldecoder
LookupError: unknown encoding: utf8-sig