Bug report

Bug description:

If you run

>>> import base64
>>> base64.b64decode("/+/+")

it gives

b'\xff\xef\xfe'

This is expected.
If you run

>>> base64.b64decode("_-_-", altchars="-_")

It gives the same output.
This is expected.
However, if you run

>>> base64.b64decode("/+/+", altchars="-_")

It also gives the same output.
This is not expected.

This is concerning because I'm trying to use file safe base64 (urlsafe_b64decode/encode do not have validate arg, which I need). I can work around this by also checking for / and + as well.

CPython versions tested on:

3.9, 3.11

Operating systems tested on:

Linux, Other

Linked PRs

• gh-125346: Add more base64 tests #141061
• [3.14] gh-125346: Add more base64 tests (GH-141061) #141130
• [3.13] gh-125346: Add more base64 tests (GH-141061) (GH-141130) #141133
• gh-125346: Fix decoding with non-standard Base64 alphabet #141128
• Remove duplicated tests in test_base64 (gh-125346) #141153