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Example Code

// See Description

Description

When submitting multiple form data with the same name, I expect the order to be always preserved when invoking the handler.

Consider the following request:

# To reliably reproduce the issue, pass a big file first and a small file second
curl https://localhost:8080/process -F image=@big_image.png -F image=@small_image.png

I expect both handlers to preserve the order of the submitted files:

# Always works correctly
async def handle_process(images: list[UploadFile]) -> ProcessingImagesResult:

# 'images' list may be in the wrong order
async def handle_process(images: t.Annotated[t.List[bytes], File()]) -> ProcessingImagesResult:

Root cause is the following change done in PR #3372 released in v0.69.0 (see my comment there):
https://github.com/fastapi/fastapi/pull/3372/changes#r2761495678

As you can see, the data order is no longer preserved after this change.

Corresponding place in the latest release (v0.128.0):

Operating System

Linux, Windows, macOS

Operating System Details

OS version does not matter

FastAPI Version

0.115.14

Pydantic Version

2.12.5

Python Version

3.12.3

Additional Context

No response