Breaking change with path parameters when updating to pydantic>=2 from pydantic<2

[Kludex]

Discussed in #11250

^{Originally posted by ThirVondukr March 5, 2024}

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Example Code

from uuid import UUID

from fastapi import FastAPI

app = FastAPI()


@app.get("/int/{path}")
async def int_(path: int | str):
    return str(type(path))


@app.get("/uuid/{path}")
async def uuid_(path: UUID | str):
    return str(type(path))

Description

With pydantic<2 parameters are correctly parsed as int and UUID if an appropriate string is passed, with pydantic 2 they're always interpreted as str, I presume this will work the same way with all other types that you can pass into path parameters.
I also observed the same behavior with query, so I assume it behaves the same way with all FastAPI parameters (headers, body, etc).

Operating System

Windows

Operating System Details

No response

FastAPI Version

0.110.0

Pydantic Version

1.10.14 / 2.6.3

Python Version

Python 3.11.5

Additional Context

No response

[H-Plus-Time]

A workaround for anyone facing this (NB: this assumes availability of typing.Annotated and pydantic.Field):

@app.get("/uuid/{path}")
-async def uuid_(path: UUID | str):
+async def uuid_(path: Annotated[UUID | str, Field(union_mode='left_to_right')]):
    return str(type(path))

Pydantic v2 switched to union_mode='smart' as the default, which in cases like this will always prefer str (since it's an exact type match).

It's also necessary to do this in any dependencies (as far as I can tell, type annotations set on direct path parameters and function parameters of dependencies aren't shared).