Description

mayeut

opened on Mar 15, 2026

Issue body actions

Bug report

Bug description:

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

import base64

assert base64.b64decode(b'/----', altchars=b'-+', ignorechars=b'/') == b'\xfb\xef\xbe'
assert base64.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

Activity

1. mayeut

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Mar 15, 2026
2. terryjreedy commented on Mar 15, 2026

   

   terryjreedy

   on Mar 15, 2026

   Member

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   @serhiy-storchaka I believe you introduced base64 ignorechars.
3. mayeut

   changed the title [-]base64.decode error raised on valid input[/-] [+]base64.b64decode error raised on valid input[/+] on Mar 15, 2026
4. serhiy-storchaka

   self-assigned this

   on Mar 15, 2026
5. mayeut commented on Mar 15, 2026

   

   mayeut

   on Mar 15, 2026

   ContributorAuthor

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   I do have a PR almost ready, I'll cc @serhiy-storchaka
6. mayeut

   added a commit that references this issue on Mar 15, 2026

   pythongh-145968: fix base64.b64decode altchars translation in specifi…

   d484023
7. StanFromIreland

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Mar 15, 2026
8. picnixz

   added

   3.15bugs and security fixesbugs and security fixes

   on Mar 15, 2026
9. serhiy-storchaka

   added a commit that references this issue on Mar 15, 2026

   gh-145968: Fix base64.b64decode altchars translation in specific cases (

   ec5e3a5
10. serhiy-storchaka commented on Mar 15, 2026

    

    serhiy-storchaka

    on Mar 15, 2026

    Member

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    Thank you for your report and PR, @mayeut.
11. serhiy-storchaka

    closed this as completedon Mar 16, 2026
12. ljfp

    added a commit that references this issue on Apr 25, 2026

    pythongh-145968: Fix base64.b64decode altchars translation in specifi…

    9534f5b

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