UnicodeError: 'IDNA does not round-trip'

[socketpair]

BPO	32437
Nosy	@vstinner, @ezio-melotti, @asvetlov, @socketpair, @berkerpeksag
Superseder	bpo-17305: IDNA2008 encoding is missing

^{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 = <Date 2017-12-28.10:19:07.562>
created_at = <Date 2017-12-28.08:48:55.445>
labels = ['3.8', 'type-bug', '3.7', 'expert-unicode']
title = "UnicodeError: 'IDNA does not round-trip'"
updated_at = <Date 2017-12-28.10:19:07.553>
user = 'https://github.com/socketpair'

bugs.python.org fields:

activity = <Date 2017-12-28.10:19:07.553>
actor = 'berker.peksag'
assignee = 'none'
closed = True
closed_date = <Date 2017-12-28.10:19:07.562>
closer = 'berker.peksag'
components = ['Unicode']
creation = <Date 2017-12-28.08:48:55.445>
creator = 'socketpair'
dependencies = []
files = []
hgrepos = []
issue_num = 32437
keywords = []
message_count = 2.0
messages = ['309117', '309118']
nosy_count = 5.0
nosy_names = ['vstinner', 'ezio.melotti', 'asvetlov', 'socketpair', 'berker.peksag']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '17305'
type = 'behavior'
url = 'https://bugs.python.org/issue32437'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8']

[socketpair]

First: This is the bug:

In [1]: 'großhandel-shop'.encode('idna')
Out[1]: b'grosshandel-shop'

This lead to this:

'xn--einla-pqa'.decode('idna')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.6/encodings/idna.py", line 214, in decode
    result.append(ToUnicode(label))
  File "/usr/lib/python3.6/encodings/idna.py", line 139, in ToUnicode
    raise UnicodeError("IDNA does not round-trip", label, label2)
UnicodeError: ('IDNA does not round-trip', b'xn--einla-pqa', b'einlass')

https://stackoverflow.com/questions/9806036/idna-does-not-round-trip

xn--grohandel-shop-2fb has been correctly encoded by IDNA 2008 (which is correct in Germany/DENIC since a while). Your Python very likely tries to decode it using the old IDNA 2003, which doesn't know 'ß'. see denic.de/en/know-how/idn-domains

[berkerpeksag]

There is an open issue about adding IDNA 2008 support: bpo-17305. Closing this one as a duplicate of that issue.