gh-74917: Raise TypeError in the JSON encoder if the default function returns the same type

[serhiy-storchaka]

Co-authored-by: Siddharth Velankar [email protected]

• Issue: json.dumps() lacks information about RecursionError related to default function #74917

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📚 Documentation preview 📚: https://cpython-previews--113458.org.readthedocs.build/

[serhiy-storchaka]

Based on #2857. Added test, Python implementation, docs.

[serhiy-storchaka]

@etrepum, could you please look at this? What are your thoughts about this matter, is it worth to do?

[etrepum]

This does catch some possible errors, but there are plenty of other ways to run into similar issues if you're not careful. For example:

>>> json.dumps(..., default=lambda x: [lambda x: x])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/homebrew/Cellar/[email protected]/3.11.6_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
          ^^^^^^^^^^^
  File "/opt/homebrew/Cellar/[email protected]/3.11.6_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 200, in encode
    chunks = self.iterencode(o, _one_shot=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/[email protected]/3.11.6_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/json/encoder.py", line 258, in iterencode
    return _iterencode(o, 0)
           ^^^^^^^^^^^^^^^^^
RecursionError: maximum recursion depth exceeded while encoding a JSON object

Perhaps a general purpose "solution" would be to catch the RecursionError and re-raise with more useful context about the value that was being serialized?

[encukou]

Fixed in #122165 instead.