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PYDANTIC FIXES GENERIC CLASS SLOT POPULATION BUG
By RepoJournal ยท Filed ยท About FastAPI & Pydantic
Pydantic patched a critical issue where `__slots__` weren't properly inherited from generic parent classes during type parameterization, breaking memory-efficient model subclasses.
The fix [1] resolves issue #13215, which exposed a gap in how Pydantic handles slot inheritance when you parameterize a generic model. If you've built memory-conscious applications using generic base classes with `__slots__`, this patch closes the hole that forced extra memory overhead on every instance. The change is surgical: Pydantic now correctly populates `__slots__` from the generic class during parameterization, ensuring that subclasses maintain the memory profile you designed for. This matters most for large-scale deployments running thousands of model instances, where the overhead compounds fast. Expect this in the next release.
Action items
- โ Watch for the next Pydantic release and test generic model subclasses with __slots__ in staging pydantic/pydantic [plan]
References
- [1] Populate `__slots__` from generic class during parameterization (#13243) pydantic/pydantic
FAQ
- What changed in FastAPI & Pydantic on May 30, 2026?
- Pydantic patched a critical issue where `__slots__` weren't properly inherited from generic parent classes during type parameterization, breaking memory-efficient model subclasses.
- What should FastAPI & Pydantic teams do about it?
- Watch for the next Pydantic release and test generic model subclasses with __slots__ in staging
- Which FastAPI & Pydantic repositories shipped on May 30, 2026?
- pydantic/pydantic