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

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Merged
serhiy-storchaka merged 3 commits into from
Mar 15, 2026
Merged

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

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d484023
gh-145968: fix base64.b64decode altchars translation in specific cases
mayeut Mar 15, 2026
081d550
simplification from review
mayeut Mar 15, 2026
e956fe6
don't use the results of an unordered 2 element set
mayeut Mar 15, 2026
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12 changes: 11 additions & 1 deletion Lib/base64.py
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Original file line number Diff line number Diff line change
Expand Up @@ -100,7 +100,17 @@ def b64decode(s, altchars=None, validate=_NOT_SPECIFIED, *, ignorechars=_NOT_SPE
break
s = s.translate(bytes.maketrans(altchars, b'+/'))
else:
trans = bytes.maketrans(b'+/' + altchars, altchars + b'+/')
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)
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@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.

s = s.translate(trans)
ignorechars = ignorechars.translate(trans)
if ignorechars is _NOT_SPECIFIED:
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7 changes: 7 additions & 0 deletions Lib/test/test_base64.py
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Original file line number Diff line number Diff line change
Expand Up @@ -293,6 +293,13 @@ def test_b64decode_altchars(self):
eq(base64.b64decode(data_str, altchars=altchars_str), res)
eq(base64.b64decode(data, altchars=altchars, ignorechars=b'\n'), res)

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

self.assertRaises(ValueError, base64.b64decode, b'', altchars=b'+')
self.assertRaises(ValueError, base64.b64decode, b'', altchars=b'+/-')
self.assertRaises(ValueError, base64.b64decode, '', altchars='+')
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-03-15-10-17-51.gh-issue-145968.gZexry.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix translation in :func:`base64.b64decode` when altchars overlaps with the
standard ones.
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