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gh-145968: fix base64.b64decode altchars translation in specific cases - #145969
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…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.
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mayeut
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Mar 15, 2026
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| 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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It can be simply:
Suggested change
| altchars_out= ( | |
| altchars[0] ifaltchars[0] notinb'+/'elsealtchars[1], | |
| altchars[1] ifaltchars[1] notinb'+/'elsealtchars[0], | |
| ) | |
| trans_in=bytearray(altchars) | |
| trans_out=bytearray(b'+/') | |
| forb, b_outinzip(b'+/', altchars_out): | |
| ifbnotinaltchars: | |
| 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'+/'))) |
ContributorAuthor
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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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You are right. What about the following?
trans_in=altcharstrans_out=b'+/'ifb'+'notinaltcharsandb'-'notinaltchars:
trans_in+=b'+/'trans_out+=altcharselifb'+'notinaltcharsorb'-'notinaltchars:
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.
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
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ljfp pushed a commit
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Apr 25, 2026
…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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When
altcharsoverlaps with the standard ones, the translation does not always yield to the expected outcome.This updates
bytes.maketransarguments to take those overlap cases into account.