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wire 2026-05-30
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PYDANTIC FIXES GENERIC CLASS SLOT POPULATION BUG

By RepoJournal · Filed · About FastAPI & Pydantic · Composed from the cited sources · methodology

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.

$ ls fastapi/reviews/ # the step-back reads

the week in review Pydantic drops Python 3.10, then layers serialization fast paths the month in review Five Pydantic validation bypasses and a quadratic URL check
Full archive → Weekly reviews →

Quick answers

What shipped 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. In total, 1 commits and 1 pull requests landed.
What were the notable FastAPI & Pydantic updates?
Populate `__slots__` from generic class during parameterization (#13243).
♥

$ log FastAPI & Pydantic --recent

Pydantic fixes GC tracking in pydantic-core and caches UTC TzInfo 50 shipped Pydantic drops Python 3.10, accepts list and DecimalTuple for Decimal 37 shipped Pydantic reworks `TypeAdapter` call arguments, tightens `Decimal` schemas 13 shipped FastAPI bumps pillow and dnspython, Pydantic corrects arguments JSON Schema 6 shipped

$ log --all # elsewhere on the wire

Agent Security Quiet day in agent security as guard coverage reaches four clients Alignment & Post-training verl defaults Ascend RL mode, open-instruct unblocks SFT packing

$ ls showcases/

Safety Evals AI Red Teaming Agent Security Interpretability Alignment & Post-training

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