Python 3.14 compatibility: AssertionError in converters due to `__args__` changes

[tsuga]

Title

Python 3.14 compatibility: AssertionError in converters due to __args__ changes

Description

Environment

• datafiles version: 2.4 (latest)
• Python version: 3.14
• OS: Ubuntu (GitHub Actions)

Problem

When using datafiles with Python 3.14, an AssertionError occurs during dataclass initialization. The error originates from converters/__init__.py where the code assumes cls.__args__ has exactly 2 elements.

Error Traceback

Traceback (most recent call last):
  File ".venv/bin/build-resume-all", line 10, in <module>
    sys.exit(build_resume_all())
  File "resume_cv_2/runner.py", line 144, in build_resume_all
    _build_resume(primary=True, others=True)
  File "resume_cv_2/runner.py", line 102, in _build_resume
    context = Content("default", Missing, Missing, Missing, Missing, Missing)
  File ".venv/lib/python3.14/site-packages/datafiles/model.py", line 84, in modified_init
    Model.__post_init__(self)
  File ".venv/lib/python3.14/site-packages/datafiles/model.py", line 20, in __post_init__
    object.__setattr__(self, "datafile", create_mapper(self))
  File ".venv/lib/python3.14/site-packages/datafiles/mapper.py", line 304, in create_mapper
    attrs[field.name] = map_type(field.type, name=field.name)
  File ".venv/lib/python3.14/site-packages/datafiles/converters/__init__.py", line 56, in map_type
    converters[field.name] = map_type(field.type, name=field.name)
  File ".venv/lib/python3.14/site-packages/datafiles/converters/__init__.py", line 64, in map_type
    assert len(cls.__args__) == 2
           ^^^^^^^^^^^^^^^^^^^^^^
AssertionError

Root Cause

Python 3.14 introduces changes to type annotation internals (related to PEP 649 and PEP 749). The __args__ attribute on generic types may have a different structure than in previous Python versions.

The assertion at converters/__init__.py:64 expects exactly 2 type arguments, but Python 3.14's type system may represent the same types differently.

Workaround

Currently using Python 3.13, which works correctly.

Additional Context

• Similar issues have been reported in other libraries using type introspection (Pydantic, Flax, etc.)

[jacebrowning]

Thanks for the detailed report! I've been meaning to add Python 3.14 support so I will get to that soon.

[jacebrowning]

This version should support Python 3.14: https://pypi.org/project/datafiles/2.5/

Let me know if you run into any issues!