bug: flet test fails with ModuleNotFoundError: No module named 'pydantic_core._pydantic_core' #7022

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1. ndonkoHenri

   self-assigned this

   on Oct 28, 2026
2. ndonkoHenri

   added

   bugSomething isn't workingSomething isn't working

   on Oct 28, 2026
3. jules-baratoux commented on Oct 28, 2026

   

   jules-baratoux

   on Oct 28, 2026

   Last edited by jules-baratoux

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   Thank you so much!

   This was on 1.0.5 from Windows.

   uvx --with flet-cli==1.0.5 flet create .
   uv add pydantic==2.14.1
   uv run flet test

   The version of pydantic seems to matter.

   Code

   Content of src/main.py

   import flet as ft
   
   
   def main(page: ft.Page):
       from pydantic import Field
       Field(eq=0)
   
   
   if __name__ == "__main__":
       ft.run(main)

   Content of tests/test_main.py

   import flet.testing as ftt
   
   from asyncio import sleep
   
   
   async def test_increment(flet_app: ftt.FletTestApp):
       await sleep(15)
       assert False

   More information

   $ uv tree --package pydantic
   Resolved 82 packages in 1ms
   pydantic v2.14.1
   ├── annotated-types v0.8.0
   ├── pydantic-core v2.50.1
   │   └── typing-extensions v4.16.0
   ├── typing-extensions v4.16.0
   └── typing-inspection v0.4.4
       └── typing-extensions v4.16.0

   I don't see pydantic-core v2.50.1 in https://pypi.flet.dev/pydantic_core/
4. ndonkoHenri commented on Oct 29, 2026

   

   ndonkoHenri

   on Oct 29, 2026

   ContributorAuthor

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   Thanks, this matches what I reproduced: same Flet 1.0.5, same pydantic 2.14.1 / pydantic-core 2.50.1, same error.

   The pydantic version shouldn't matter on Windows: flet test builds the app in Flutter's Debug mode, and on Windows that embeds a debug build of Python, which can only load compiled modules named *_d.pyd. The pydantic-core wheel from PyPI is installed correctly, but its .pyd is the regular release build, so the debug Python never loads it. Any package with compiled code fails the same way. If an older version worked for you, was it pydantic 1.x? It can run without its compiled module.

   pypi.flet.dev only hosts Android and iOS wheels, so it isn't involved here. Windows uses the wheels from PyPI, and flet build already adds pypi.flet.dev for mobile builds. For Android or iOS you'd currently need pydantic==2.13.4, the newest release whose pydantic-core has mobile wheels there.

   Fixes:

   • Load the release CPython in Windows Debug builds serious-python#261 makes Debug builds use the regular Python, so compiled packages import under flet test. Verified with your pydantic version.
   • Report Python startup errors in flet test; bump Flutter to 3.47.8 #7023 makes flet test fail right away with the Python traceback when the app can't start, so you won't need the sleep to see the error.

   ❤️React with ❤️1Reacted by Jules Baratoux
5. jules-baratoux commented on Oct 29, 2026

   

   jules-baratoux

   on Oct 29, 2026

   More actions

   Thank you again for your prompt help and explanations. Unfortunately, I'm afraid I don't remember which version worked last night while I was trying to pin down the issue for reproduction.