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gh-53891: Include label value in IDNA 'label too long' errors #144855

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gh-53891: Include label value in IDNA 'label too long' errors #144855

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11 changes: 7 additions & 4 deletions Lib/encodings/idna.py
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Original file line number Diff line number Diff line change
Expand Up @@ -77,7 +77,8 @@ def ToASCII(label): # type: (str) -> bytes
if len(label) == 0:
raise UnicodeEncodeError("idna", label, 0, 1, "label empty")
else:
raise UnicodeEncodeError("idna", label, 0, len(label), "label too long")
raise UnicodeEncodeError("idna", label, 0, len(label),
f"label too long: {label!r}")

# Step 2: nameprep
label = nameprep(label)
Expand All @@ -95,7 +96,8 @@ def ToASCII(label): # type: (str) -> bytes
if len(label) == 0:
raise UnicodeEncodeError("idna", label, 0, 1, "label empty")
else:
raise UnicodeEncodeError("idna", label, 0, len(label), "label too long")
raise UnicodeEncodeError("idna", label, 0, len(label),
f"label too long: {label!r}")

# Step 5: Check ACE prefix
if label.lower().startswith(sace_prefix):
Expand All @@ -112,7 +114,8 @@ def ToASCII(label): # type: (str) -> bytes
# do not check for empty as we prepend ace_prefix.
if len(label_ascii) < 64:
return label_ascii
raise UnicodeEncodeError("idna", label, 0, len(label), "label too long")
raise UnicodeEncodeError("idna", label, 0, len(label),
f"label too long: {label!r}")

def ToUnicode(label):
if len(label) > 1024:
Expand Down Expand Up @@ -201,7 +204,7 @@ def encode(self, input, errors='strict'):
if len(label) >= 64:
offset = sum(len(l) for l in labels[:i]) + i
raise UnicodeEncodeError("idna", input, offset, offset+len(label),
"label too long")
f"label too long: {label.decode('ascii')!r}")
return result, len(input)

result = bytearray()
Expand Down
23 changes: 23 additions & 0 deletions Lib/test/test_codecs.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1640,6 +1640,29 @@ def test_builtin_decode_length_limit(self):
with self.assertRaisesRegex(UnicodeDecodeError, "too long"):
(b"xn--016c"+b"a"*70).decode("idna")

def test_error_message_label_too_long(self):
# gh-53891: "label too long" errors should include the offending label
long_label = "a" * 70
domain = f"{long_label}.com"
with self.assertRaises(UnicodeEncodeError) as cm:
domain.encode("idna")
self.assertIn(long_label, cm.exception.reason)
self.assertIn("label too long", cm.exception.reason)

def test_error_message_label_too_long_non_ascii(self):
# gh-53891: non-ASCII labels should also be included in the message
long_label = "\xe4" * 70
domain = f"{long_label}.com"
with self.assertRaises(UnicodeEncodeError) as cm:
domain.encode("idna")
self.assertIn("label too long", cm.exception.reason)

def test_error_message_label_empty(self):
# gh-53891: empty label errors should have a clear reason
with self.assertRaises(UnicodeEncodeError) as cm:
"a..com".encode("idna")
self.assertEqual(cm.exception.reason, "label empty")

def test_stream(self):
r = codecs.getreader("idna")(io.BytesIO(b"abc"))
r.read(3)
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-02-15-23-25-41.gh-issue-53891.JlZccA.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Include the offending label value in IDNA codec "label too long" error
messages to make them more informative and easier to debug.
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