[cristianchiriac]

Closes #983

UnionField.generate_unions_type checked type(self.default) is not dataclasses._MISSING_TYPE. Python 3.15 made dataclasses.MISSING a PEP 661 sentinel and removed the private _MISSING_TYPE class, so generating the schema of any union field raised AttributeError (the issue's traceback).

The check is now self.default is not dataclasses.MISSING. MISSING is a singleton on all supported versions, and the rest of the package (including the if right above this line) already compares with it this way.

Tested with poetry install --extras "pydantic faust faker" + pytest tests, in Docker:

	before	after
python:3.14		2964 passed
python:3.15	197 failed, 2767 passed	35 failed, 2929 passed

The issue's reproducer prints {"type": "record", "name": "Class", "fields": [{"name": "field", "type": ["string", "long"]}]} on 3.15.

The 35 remaining 3.15 failures are all in the faust tests and come from a dependency: mode/utils/objects.py calls typing._eval_type() without the type_params argument that 3.15 now requires. Nothing in this repo is involved, so I left those alone and didn't add 3.15 to the CI matrix.

dataclasses._MISSING_TYPE is still used in three return annotations (fields.py lines 200, 282, 571). Those don't run at runtime on 3.14+ (annotations are lazy), and typeshed still declares the class, so I didn't change them here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.76%. Comparing base (830d6bd) to head (9c8c8e8).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #984      +/-   ##
==========================================
+ Coverage   98.72%   98.76%   +0.04%     
==========================================
  Files          32       32              
  Lines        2271     2271              
==========================================
+ Hits         2242     2243       +1     
+ Misses         29       28       -1     

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