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GeezerVison is showing github.com — gh-145968: fix base64.b64decode altchars translation in specific cases by mayeut · Pull Request #145969 · python/cpython

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@mayeut

@mayeut mayeut commented Mar 15, 2026 •
edited by bedevere-app Bot
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When altchars overlaps with the standard ones, the translation does not always yield to the expected outcome.
This updates bytes.maketrans arguments to take those overlap cases into account.

  • Issue: base64.b64decode error raised on valid input #145968

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@mayeut
pythongh-145968: fix base64.b64decode altchars translation in specifi…
d484023 
…c cases

When `altchars` overlaps with the standard ones, the translation does not always yield to the expected outcome.
This commit updates `bytes.maketrans` arguments to take those overlap cases into account.
@python-cla-bot

python-cla-bot Bot commented Mar 15, 2026 •
edited
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@bedevere-app bedevere-app Bot mentioned this pull request Mar 15, 2026
base64.b64decode error raised on valid input #145968
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@bedevere-app bedevere-app Bot added the awaiting review label Mar 15, 2026
@mayeut

mayeut commented Mar 15, 2026

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cc @serhiy-storchaka
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serhiy-storchaka
serhiy-storchaka reviewed Mar 15, 2026
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Comment thread Lib/base64.py Outdated
Comment on lines +103 to +113
altchars_out = (
altchars[0] if altchars[0] not in b'+/' else altchars[1],
altchars[1] if altchars[1] not in b'+/' else altchars[0],
)
trans_in = bytearray(altchars)
trans_out = bytearray(b'+/')
for b, b_out in zip(b'+/', altchars_out):
if b not in altchars:
trans_in.append(b)
trans_out.append(b_out)
trans = bytes.maketrans(trans_in, trans_out)

@serhiy-storchaka serhiy-storchaka Mar 15, 2026

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It can be simply:

Suggested change
altchars_out = (
altchars[0] if altchars[0] not in b'+/' else altchars[1],
altchars[1] if altchars[1] not in b'+/' else altchars[0],
)
trans_in = bytearray(altchars)
trans_out = bytearray(b'+/')
for b, b_out in zip(b'+/', altchars_out):
if b not in altchars:
trans_in.append(b)
trans_out.append(b_out)
trans = bytes.maketrans(trans_in, trans_out)
trans = bytes.maketrans(altchars + bytes(set(b'+/') - set(altchars)),
b'+/' + bytes(set(altchars) - set(b'+/')))

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@mayeut mayeut Mar 15, 2026 •
edited
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I commited the suggestion but after further reflection, the sets are unordered and thus the translation might not be correct when altchars and standard ones do not overlap (although this might be hard to catch in tests ?).
I'll fix that later

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@serhiy-storchaka serhiy-storchaka Mar 15, 2026

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You are right. What about the following?

trans_in = altchars
trans_out = b'+/'
if b'+' not in altchars and b'-' not in altchars:
    trans_in += b'+/'
    trans_out += altchars
elif b'+' not in altchars or b'-' not in altchars:
    trans_in += bytes(set(b'+/') - set(altchars))
    trans_out += bytes(set(altchars) - set(b'+/'))

You can also return your original code. #145981 also fixes this issue, so this code can be replaced, but your tests will remain.

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mayeut and others added 2 commits March 15, 2026 11:36
@mayeut @serhiy-storchaka
simplification from review
081d550 
Co-authored-by: Serhiy Storchaka <[email protected]>
@mayeut
don't use the results of an unordered 2 element set
e956fe6
serhiy-storchaka
serhiy-storchaka approved these changes Mar 15, 2026
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@serhiy-storchaka serhiy-storchaka left a comment

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LGTM. 👍

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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting review labels Mar 15, 2026
@serhiy-storchaka
serhiy-storchaka merged commit ec5e3a5 into python:main Mar 15, 2026
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@bedevere-app bedevere-app Bot removed the awaiting merge label Mar 15, 2026
@mayeut
mayeut deleted the base64-bad-translation branch March 29, 2026 17:21
ljfp pushed a commit to ljfp/cpython that referenced this pull request Apr 25, 2026
@mayeut @ljfp
pythongh-145968: Fix base64.b64decode altchars translation in specifi…
9534f5b 
…c cases (pythonGH-145969)

When altchars overlaps with the standard ones, the translation does not
always yield to the expected outcome.
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@serhiy-storchaka serhiy-storchaka serhiy-storchaka approved these changes

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2 participants

@mayeut @serhiy-storchaka
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