What were the notable FastAPI & Pydantic updates?

Populate `__slots__` from generic class during parameterization (#13243).

FastAPI & Pydantic

FastAPI and the Pydantic + SQLModel async-Python stack

Pick a date

Topics: Python Full archive →

The Wire · Showcase

PYDANTIC FIXES GENERIC CLASS SLOT POPULATION BUG

By RepoJournal · Filed 06:04 UTC on May 30, 2026 · 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

→ 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

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).

FASTAPI 0.140.0 CUTS MEMORY OVERHEAD IN DEPENDENCIES

FastAPI shipped a memory optimization that simplifies how dependencies are processed, backed by months of community analysis and benchmarking.

python 9 shipped 1-min read

FastAPI & Pydantic 1 day ago

FASTAPI AND SQLMODEL STANDARDIZE ON UV, PYDANTIC DOCS BLOCKED ON SETTINGS DRIFT

Tiangolo shipped Library Skills documentation across FastAPI and SQLModel while pivoting both projects toward uv as the default package manager, but Pydantic's docs build is failing due to version mismatch in pydantic-settings.

python 10 shipped 1-min read

FastAPI & Pydantic 2 days ago

SQLMODEL FIXES TERMYNAL SCROLL JAR ACROSS DOCS

Interactive code examples in SQLModel documentation no longer hijack your scroll position when you click the restart or fast-forward controls.

python 2 shipped 1-min read

FastAPI & Pydantic 3 days ago

FASTAPI AND PYDANTIC SHIFT TO UV FOR DOCS AND DEPENDENCIES

FastAPI's documentation is standardizing on uv project templates, signaling a broader ecosystem shift toward Python's fastest-growing package manager.

python 7 shipped 1-min read

Elsewhere on the wire

AI Agents about 9 hours ago

CLAUDE OPUS 5 LANDS ACROSS THE STACK

The newest Anthropic model is now live in langchain, Cline, and llama-index, with native support for extended reasoning and 1M context windows.

ai-agents 28 shipped 1-min read

Local LLMs about 9 hours ago

OLLAMA LANDS LAGUNA SUPPORT AND CRUSHES MEMORY LEAKS WHILE SGLANG HITS V0.5.16 WITH CONFIDENCE-DRIVEN SPECULATIVE DECODING

Ollama shipped three critical performance and reliability fixes for Metal residency and concurrent access patterns, while SGL-Lang released 0.5.16 with a new speculative algorithm hitting 383.7 tok/s on DeepSeek-V4.