base64 documentation is out-dated; altchars has to have the exact length of 2.

[ynn]

BPO	46559

^{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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assignee = None
closed_at = None
created_at = <Date 2022-01-28.02:52:31.566>
labels = ['3.10', 'docs']
title = 'base64 documentation is out-dated; altchars has to have the exact length of 2.'
updated_at = <Date 2022-01-28.02:52:31.566>
user = 'https://bugs.python.org/ynn'

bugs.python.org fields:

activity = <Date 2022-01-28.02:52:31.566>
actor = 'ynn'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2022-01-28.02:52:31.566>
creator = 'ynn'
dependencies = []
files = []
hgrepos = []
issue_num = 46559
keywords = []
message_count = 1.0
messages = ['411967']
nosy_count = 2.0
nosy_names = ['docs@python', 'ynn']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue46559'
versions = ['Python 3.10']

Linked PRs

• gh-90717: Update the documentation for the altchars paremeter in base64 library #94187
• [3.11] gh-90717: Update the documentation for the altchars paremeter in base64 library (GH-94187) #99878
• [3.10] gh-90717: Update the documentation for the altchars paremeter in base64 library (GH-94187) #99879

[ynn]

According to the documentation of base64 (https://docs.python.org/3/library/base64.html),

Optional altchars must be a bytes-like object of at least length 2 (additional characters are ignored) which specifies an alternative alphabet for the + and / characters.

but this explanation is out-dated.

Actually this had been correct until the commit 4581ae5fa2450db3f00384e4b2e86654605100d4 was made for cpython in "2007".
(Thus, the explanation has been incorrect for about 15 years.)

The current implementation requires the length of altchars to be exactly two, as explicitly written in the comment:

Optional altchars should be a byte string of length 2 which specifies an alternative alphabet for the '+' and '/' characters.

and the corresponding assertion is:

assert len(altchars) == 2, repr(altchars)