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Fix email header folding exceeding max_line_length on continuation lines #3

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Fix email header folding exceeding max_line_length on continuation lines #3

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7 changes: 7 additions & 0 deletions Lib/email/_header_value_parser.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3001,6 +3001,13 @@ def _fold_as_ew(to_encode, lines, maxlen, last_ew, ew_combine_allowed, charset,
encoded_word = _ew.encode(leading_whitespace, charset=encode_as)
lines[-1] += encoded_word
leading_whitespace = ''
# The leading whitespace was encoded as its own encoded word and
# appended to the line, so recompute the space left on the line.
remaining_space = maxlen - len(lines[-1])
text_space = remaining_space - chrome_len
if text_space <= 0:
lines.append(' ')
Comment on lines +3006 to +3009

@chatgpt-codex-connector chatgpt-codex-connector Bot Jun 7, 2026

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P2 Badge Start a new line when no character fits after the whitespace word

When the recomputed text_space is positive but too small for the encoded representation of the first character, the shrinking loop below reduces to_encode_word to '' and emits an invalid empty encoded-word. For example, folding the new regression input with max_line_length=28 now produces =?utf-8?q?_?==?utf-8?q??=. RFC 2047 requires nonempty encoded text; detect the empty candidate and move the actual word to a new continuation line instead.

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continue

to_encode_word = to_encode[:text_space]
encoded_word = _ew.encode(to_encode_word, charset=encode_as)
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15 changes: 15 additions & 0 deletions Lib/test/test_email/test__header_value_parser.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3123,6 +3123,21 @@ def test_unknown_after_unknown(self):
"=?unknown-8bit?q?=A4?="),
prefix + "=?unknown-8bit?q?=C2?=\n =?unknown-8bit?q?=A4?=\n")

def test_long_ew_after_encoded_continuation_whitespace(self):
# When an encoded word begins a continuation line, the line's leading
# whitespace is emitted as its own encoded word. The space it consumes
# must be subtracted from the budget, otherwise the following encoded
# word overflows max_line_length.
policy = self.policy.clone(max_line_length=40)
self._test(parser.get_unstructured(
'a b c d ä d .,ä, dc cbaö,ä.,baaöa üa.,ü,c,äöäa,bööüc üü'),
'a b c d =?utf-8?b?w6QgZCAuLMOkLA==?= dc\n'
' =?utf-8?q?_?==?utf-8?b?Y2Jhw7Ysw6Qu?=\n'
' =?utf-8?b?LGJhYcO2YSDDvGEuLMO8LGMs?=\n'
' =?utf-8?b?w6TDtsOkYSxiw7bDtsO8YyA=?=\n'
' =?utf-8?q?=C3=BC=C3=BC?=\n',
policy=policy)

# XXX Need test of an encoded word so long that it needs to be wrapped

def test_simple_address(self):
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