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🐛 Fix internal Pydantic v1 compatibility (warnings) for Python 3.14 and Pydantic 2.12.1 #14186

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tiangolo merged 25 commits into from
Oct 20, 2025
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🐛 Fix internal Pydantic v1 compatibility (warnings) for Python 3.14 and Pydantic 2.12.1 #14186

tiangolo merged 25 commits into from
Oct 20, 2025
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@svlandeg svlandeg commented Oct 14, 2025 •
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The Python 3.14 tests started failing with the Pydantic v.2.12.1 release, because of the combination of #14168 and pydantic/pydantic#12367.

Basically we were importing Pydantic v1 functionality even when the user had (only) Pydantic v2 installed. This was done in #14168 to support more easy migration from v1 to v2 by allowing mixing versions within the same app.

Here, we prevent importing any v1 functionality by creating dummy classes for Python 3.14 / Pydantic 2.12.0 onwards, which drops support for v1 and emits a warning when importing v1 functionality.

TODO

  • fix coverage
  • check warning is still shown to users at some point, and not just swallowed by this code.

@github-actions github-actions bot added the docs Documentation about how to use FastAPI label Oct 14, 2025
@svlandeg svlandeg added feature New feature or request and removed docs Documentation about how to use FastAPI labels Oct 14, 2025
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@svlandeg svlandeg added bug Something isn't working and removed feature New feature or request labels Oct 14, 2025
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github-actions bot commented Oct 14, 2025 •
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📝 Docs preview

Last commit b7d4d4c at: https://ffdce6a4.fastapitiangolo.pages.dev

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@svlandeg svlandeg changed the title Test CI (WIP) 🐛 Fix Pydantic v1 compatibility for Python 3.14 and Pydantic 2.12.1 Oct 14, 2025
@svlandeg svlandeg self-assigned this Oct 14, 2025
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svlandeg commented Oct 15, 2025
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@svlandeg svlandeg marked this pull request as ready for review October 15, 2025 13:55
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tiangolo
tiangolo approved these changes Oct 20, 2025
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Awesome, thank you @svlandeg! 🙌

I checked and confirmed that on Python 3.14, if users try to use Pydantic v1 they still get a warning on their own code, but not on older versions. So we're good to go! 🚀

@tiangolo tiangolo changed the title 🐛 Fix Pydantic v1 compatibility for Python 3.14 and Pydantic 2.12.1 🐛 Fix internal Pydantic v1 compatibility (warnings) for Python 3.14 and Pydantic 2.12.1 Oct 20, 2025
@tiangolo tiangolo merged commit d8c691f into fastapi:master Oct 20, 2025
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@tiangolo tiangolo mentioned this pull request Oct 20, 2025
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@svlandeg svlandeg deleted the fix/ci branch October 20, 2025 12:23
@svlandeg svlandeg removed their assignment Oct 21, 2025
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