Openapi.json does not support pydantic v1 and pydantic v2 at the same time

[ellyx13]

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Example Code

from typing import Union
from fastapi import FastAPI
from pydantic import v1 as pydantic_v1
from pydantic import BaseModel

app = FastAPI()

class Foo(pydantic_v1.BaseModel):
    message: str

class Bar(BaseModel):
    message: str



@app.post("/foo")
def read_item(data: Foo):
    data = data.dict()
    return {"message": data['message']}

@app.post("/bar")
def read_item(data: Bar):
    data = data.dict()
    return {"message": data['message']}

# Run the server with: uvicorn main:app --reload

Description

I am using FastAPI 0.110.3 and pydantic v2.7.1. Previously I used pydantic v1 and am planning to migrate to pydantic v2. But I have a problem that FastAPI cannot declare 2 api at the same time using BaseModel of pydantic v1 and pydantic v2.

I did some research and found out that pydantic v1 uses openapi.json 3.0 and pydantic v2 uses openapi.json 3.1, so I think the two versions of openapi.json conflict with each other.

I want fastapi to load the api using pydantic v1 in /v1/api path. And the api using pydantic v2 will be in the v2/api path. But I get error

raise PydanticInvalidForJsonSchema(f'Cannot generate a JsonSchema for {error_info}')
pydantic.errors.PydanticInvalidForJsonSchema: Cannot generate a JsonSchema for core_schema.PlainValidatorFunctionSchema ({'type': 'with-info', 'function': <bound method BaseModel.validate of <class 'main.Foo'>>})

Operating System

Linux

Operating System Details

pydantic version: 2.7.1
pydantic-core version: 2.18.2
pydantic-core build: profile=release pgo=true
install path: /home/kietpham/python/pydantic/venv/lib/python3.10/site-packages/pydantic
python version: 3.10.12 (main, Nov 20 2023, 15:14:05) [GCC 11.4.0]
platform: Linux-5.15.146.1-microsoft-standard-WSL2-x86_64-with-glibc2.35
related packages: fastapi-0.110.3 typing_extensions-4.11.0
commit: unknown

FastAPI Version

0.110.3

Pydantic Version

2.7.1

Python Version

Python 3.12.3

Additional Context

Library version from pip freeze:
annotated-types==0.6.0
anyio==4.3.0
click==8.1.7
exceptiongroup==1.2.1
fastapi==0.110.3
h11==0.14.0
httptools==0.6.1
idna==3.7
pydantic==2.7.1
pydantic_core==2.18.2
python-dotenv==1.0.1
PyYAML==6.0.1
sniffio==1.3.1
starlette==0.37.2
typing_extensions==4.11.0
uvicorn==0.29.0
uvloop==0.19.0
watchfiles==0.21.0
websockets==12.0

[kyogom]

Hi, ellyx13.
I reproduced your problem.

I did some research and found out that pydantic v1 uses openapi.json 3.0 and pydantic v2 uses openapi.json 3.1, so I think the two versions of openapi.json conflict with each other.

And I set openapi version by extending openapi but it doesn't fix error.

....

@app.post("/bar")
def read_item(data: Bar):
    data = data.dict()
    return {"message": data["message"]}


def custom_openapi():
    if app.openapi_schema:
        return app.openapi_schema
    openapi_schema = get_openapi(
        title="title",
        version="1.0.0",
        openapi_version="3.0.0", # set to 3.0.0
        routes=app.routes,
    )
    app.openapi_schema = openapi_schema
    return app.openapi_schema


app.openapi = custom_openapi

Fastapi seems refer pydantic.version and currently it doesn't seem possible to create openapi document using pydantic model v1 and v2 at the same time.

fastapi/fastapi/_compat.py

Line 23 in e04d397

from pydantic.version import VERSION as P_VERSION