String enums in dicts/lists are json serialized differently in Python 3.15

Bug report

Bug description:

Cross posting from cattrs: python-attrs/cattrs#744
There's a difference in how Python 3.14 and Pytthon 3.15 handle json serialization of string enum values stored in other types which is exposed by cattrs tests.
I have failed to find any track record for this behavior in the available docs.

See:

Python 3.15.0a7

>>> import json
>>> from enum import Enum
>>> class AStringEnum(str, Enum):
...     A = "a"
...     
>>> val = AStringEnum.A
>>> json.dumps(val)
'"a"'
>>> in_dict = json.dumps({"key": val})
>>> in_dict
'{"key": "AStringEnum.A"}'
>>> parsed = json.loads(in_dict)
>>> type(parsed['key'])
<class 'str'>
>>> repr(parsed['key'])
"'AStringEnum.A'"
>>> AStringEnum(parsed['key'])
Traceback (most recent call last):
  File "<python-input-15>", line 1, in <module>
    AStringEnum(parsed['key'])
    ~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.15/enum.py", line 732, in __call__
    return cls.__new__(cls, value)
           ~~~~~~~~~~~^^^^^^^^^^^^
  File "/usr/lib64/python3.15/enum.py", line 1221, in __new__
    raise ve_exc
ValueError: 'AStringEnum.A' is not a valid AStringEnum

Python 3.14.3

>>> import json
>>> from enum import Enum
>>> class AStringEnum(str, Enum):
...     A = "a"
...  
>>> val = AStringEnum.A
>>> json.dumps(val)
'"a"'
>>> json.dumps({"key": val})
'{"key": "a"}'
>>> in_dict = json.dumps({"key": val})
>>> parsed = json.loads(in_dict)
>>> type(parsed['key'])
<class 'str'>
>>> repr(parsed['key'])
"'a'"
>>> AStringEnum(parsed['key'])
<AStringEnum.A: 'a'>

CPython versions tested on:

3.15

Operating systems tested on:

Linux

Linked PRs

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

@nineteendo

Reproducible with 3.15.0a1, and bisects to dec624e (PR #133239, issue #135336): "Add fast path to json string encoding".

cc @nineteendo @vstinner

This is because the __str__ changed, on main:

>>> str(parsed['key'])
'AStringEnum.A'

whereas previously:

>>> str(parsed['key'])
'a'

Code to write the dictionary key:

Python 3.14: encoder_listencode_obj() => encoder_encode_string() => py_encode_basestring_ascii() => ascii_escape_unicode(). This function copies the string character by character using PyUnicode_READ().
Python 3.15: encoder_listencode_obj() => encoder_writer_string() => write_escaped_ascii(). This function calls PyUnicodeWriter_WriteStr() which calls PyObject_Str() since the dictionary key is a str subclass and not exactly the str type. PyObject_Str() returns AStringEnum.A string.

So the difference is in write_escaped_ascii() which now calls PyObject_Str() for str subclasses.

Example:

import enum
import _testlimitedcapi

unicode_fromobject = _testlimitedcapi.unicode_fromobject

class AStringEnum(str, enum.Enum):
    A = "a"

obj = AStringEnum.A
print(f"str: {str(obj)!r}")
print(f"PyUnicode_FromObject: {unicode_fromobject(obj)!r}")

Output on Python 3.15:

str: 'AStringEnum.A'
PyUnicode_FromObject: 'a'

pythongh-148241: Fix json serialization for str subclasses

I created PR gh-148249 to fix the json regression.

I also created issue gh-148250 to discuss PyUnicodeWriter_WriteStr() usage with str subclasses.

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

I close the issue, it has been fixed by the change b7e1d51. Thanks for the bug report!

pythongh-148241: Fix json serialization for str subclasses (python#14…

Address review feedback for the streaming JSON encoder

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String enums in dicts/lists are json serialized differently in Python 3.15 #148241

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String enums in dicts/lists are json serialized differently in Python 3.15 #148241

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Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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hugovk commented on Apr 8, 2026

StanFromIreland commented on Apr 8, 2026

vstinner commented on Apr 8, 2026

vstinner commented on Apr 8, 2026

vstinner commented on Apr 8, 2026

vstinner commented on Apr 9, 2026

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