Bug report

Bug description:

An error is raised for the following valid test cases using the newly introducedignorechars parameter

importbase64assertbase64.b64decode(b'/----',altchars=b'-+',ignorechars=b'/')==b'\xfb\xef\xbe'assertbase64.b64decode(b'+----',altchars=b'/-',ignorechars=b'+')==b'\xff\xff\xff'

For both we get:

binascii.Error: Invalid base64-encoded string: number of data characters (5) cannot be 1 more than a multiple of 4

In both cases, the leading byte should be ignored thus giving a 4 bytes valid input.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-145968: fix base64.b64decode altchars translation in specific cases #145969