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gh-148241: Fix json serialization for str subclasses #148249

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Merged
vstinner merged 3 commits into from
Apr 9, 2026
Merged

gh-148241: Fix json serialization for str subclasses #148249

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1f892cb
gh-148241: Fix json serialization for str subclasses
vstinner Apr 8, 2026
c251659
Add a test to test_json.test_enum
vstinner Apr 8, 2026
fdab889
Reformat tests
vstinner Apr 8, 2026
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38 changes: 38 additions & 0 deletions Lib/test/test_json/test_dump.py
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Original file line number Diff line number Diff line change
Expand Up @@ -77,6 +77,44 @@ def __lt__(self, o):
d[1337] = "true.dat"
self.assertEqual(self.dumps(d, sort_keys=True), '{"1337": "true.dat"}')

def test_dumps_str_subclass(self):
# Don't call obj.__str__() on str subclasses

# str subclass which returns a different string on str(obj)
class StrSubclass(str):
def __str__(self):
return "StrSubclass"

obj = StrSubclass('ascii')
self.assertEqual(self.dumps(obj),
'"ascii"')
self.assertEqual(self.dumps([obj]),
'["ascii"]')
self.assertEqual(self.dumps({'key': obj}),
'{"key": "ascii"}')

obj = StrSubclass('escape\n')
self.assertEqual(self.dumps(obj),
'"escape\\n"')
self.assertEqual(self.dumps([obj]),
'["escape\\n"]')
self.assertEqual(self.dumps({'key': obj}),
'{"key": "escape\\n"}')

obj = StrSubclass('nonascii:é')
self.assertEqual(self.dumps(obj, ensure_ascii=False),
'"nonascii:é"')
self.assertEqual(self.dumps([obj], ensure_ascii=False),
'["nonascii:é"]')
self.assertEqual(self.dumps({'key': obj}, ensure_ascii=False),
'{"key": "nonascii:é"}')
self.assertEqual(self.dumps(obj),
'"nonascii:\\u00e9"')
self.assertEqual(self.dumps([obj]),
'["nonascii:\\u00e9"]')
self.assertEqual(self.dumps({'key': obj}),
'{"key": "nonascii:\\u00e9"}')


class TestPyDump(TestDump, PyTest): pass

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7 changes: 7 additions & 0 deletions Lib/test/test_json/test_encode_basestring_ascii.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,11 @@
from test.support import bigaddrspacetest


# str subclass which returns a different string on str(obj)
class StrSubclass(str):
def __str__(self):
return "StrSubclass"

CASES = [
('/\\"\ucafe\ubabe\uab98\ufcde\ubcda\uef4a\x08\x0c\n\r\t`1~!@#$%^&*()_+-=[]{}|;:\',./<>?', '"/\\\\\\"\\ucafe\\ubabe\\uab98\\ufcde\\ubcda\\uef4a\\b\\f\\n\\r\\t`1~!@#$%^&*()_+-=[]{}|;:\',./<>?"'),
('\u0123\u4567\u89ab\ucdef\uabcd\uef4a', '"\\u0123\\u4567\\u89ab\\ucdef\\uabcd\\uef4a"'),
Expand All @@ -14,6 +19,8 @@
('\U0001d120', '"\\ud834\\udd20"'),
('\u03b1\u03a9', '"\\u03b1\\u03a9"'),
("`1~!@#$%^&*()_+-={':[,]}|;.</>?", '"`1~!@#$%^&*()_+-={\':[,]}|;.</>?"'),
# Don't call obj.__str__() on str subclasses
(StrSubclass('ascii'), '"ascii"'),
]

class TestEncodeBasestringAscii:
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-04-08-14-19-17.gh-issue-148241.fO_QT4.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
:mod:`json`: Fix serialization: no longer call ``str(obj)`` on :class:`str`
subclasses. Patch by Victor Stinner.
9 changes: 7 additions & 2 deletions Modules/_json.c
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Original file line number Diff line number Diff line change
Expand Up @@ -258,7 +258,10 @@ write_escaped_ascii(PyUnicodeWriter *writer, PyObject *pystr)
if (PyUnicodeWriter_WriteChar(writer, '"') < 0) {
return -1;
}
if (PyUnicodeWriter_WriteStr(writer, pystr) < 0) {
// gh-148241: Avoid PyUnicodeWriter_WriteStr() which calls str(obj)
// on str subclasses
assert(PyUnicode_IS_ASCII(pystr));
if (PyUnicodeWriter_WriteASCII(writer, input, input_chars) < 0) {
return -1;
}
return PyUnicodeWriter_WriteChar(writer, '"');
Expand Down Expand Up @@ -399,7 +402,9 @@ write_escaped_unicode(PyUnicodeWriter *writer, PyObject *pystr)
if (PyUnicodeWriter_WriteChar(writer, '"') < 0) {
return -1;
}
if (PyUnicodeWriter_WriteStr(writer, pystr) < 0) {
// gh-148241: Avoid PyUnicodeWriter_WriteStr() which calls str(obj)
// on str subclasses
if (_PyUnicodeWriter_WriteStr((_PyUnicodeWriter*)writer, pystr) < 0) {
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@vstinner vstinner Apr 8, 2026

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An alternative to using the private API (which is bad!) is to call PyUnicode_FromObject(). But PyUnicode_FromObject() has to copy the string, it's less efficient.

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@vstinner vstinner Apr 8, 2026

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See also the issue gh-148250 that I just created for this issue.

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@sunmy2019 sunmy2019 Apr 8, 2026

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using the private API

It would raise a deprecated warning.

_Py_DEPRECATED_EXTERNALLY(3.14) PyAPI_FUNC(int) _PyUnicodeWriter_WriteStr(
    _PyUnicodeWriter *writer,
    PyObject *str);

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@serhiy-storchaka serhiy-storchaka Apr 8, 2026

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Is it safe to cast PyUnicodeWriter* to _PyUnicodeWriter*?

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@vstinner vstinner Apr 8, 2026

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Yes, it's safe. That's how PyUnicodeWriter_WriteStr() is implemented: by casting the first argument to _PyUnicodeWriter* and calling _PyUnicodeWriter_WriteStr().

Do you prefer calling PyUnicode_FromObject() to avoid the private API?

It would raise a deprecated warning.

_Py_DEPRECATED_EXTERNALLY() only emits a deprecation warning if the Py_BUILD_CORE macro is not defined. This macro is defined in Modules/_json.c, so no warning is emitted.

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@sunmy2019 sunmy2019 Apr 8, 2026

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Is it safe to cast PyUnicodeWriter* to _PyUnicodeWriter*?

Yes. I checked, PyUnicodeWriter is an opaque type that is always cast to _PyUnicodeWriter before use.

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@sunmy2019 sunmy2019 Apr 8, 2026

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I am in favor of public API.

Is PyUnicodeWriter_WriteSubstring applicable for this?

PyUnicodeWriter_WriteSubstring the only PyUnicodeWriter public API that treats str subclasses as str.

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@serhiy-storchaka serhiy-storchaka Apr 8, 2026

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It is fine. We already use private API in this file.

return -1;
}
return PyUnicodeWriter_WriteChar(writer, '"');
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