validation errors for openAPI schema with length >=2 tuples

First Check

• I added a very descriptive title to this issue.
• I used the GitHub search to find a similar issue and didn’t find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google “How to X in FastAPI” and didn’t find any information.
• I already read and followed all the tutorial in the docs and didn’t find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Commit to Help

• I commit to help with one of those options 👆

Example Code

from typing import Tuple
from fastapi import FastAPI
from pydantic import BaseModel


class FooArgs(BaseModel):
    bars: Tuple[str, str]


app = FastAPI()


@app.post("/foo")
def foo(args: FooArgs):
    ...

Description

If you try to retrieve the /openapi.json, the FooArgs get a validation error:

  File ".../lib/python3.9/site-packages/fastapi/openapi/utils.py", line 410, in get_openapi
    return jsonable_encoder(OpenAPI(**output), by_alias=True, exclude_none=True)  # type: ignore
  File "pydantic/main.py", line 406, in pydantic.main.BaseModel.__init__
pydantic.error_wrappers.ValidationError: 2 validation errors for OpenAPI
components -> schemas -> FooArgs -> properties -> bars -> items
  value is not a valid dict (type=type_error.dict)
components -> schemas -> FooArgs -> $ref
  field required (type=value_error.missing)

This error is new to fastapi>=0.68, it does not happen with fastapi==0.67.0.

I think this is related to Pydantic coercing Tuple[str, str] into Dict[str, str] somewhere?

Things that don’t cause this error:

class FooArgs(BaseModel):
    bars: Tuple[str]

class FooArgs(BaseModel):
    bars: Tuple[str, ...]

This that do cause this error:

class FooArgs(BaseModel):
    bars: Tuple[str, str]

class FooArgs(BaseModel):
    bars: Tuple[int, str]

class FooArgs(BaseModel):
    bars: Tuple[str, str, str]  # etc.

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.68.1

Python Version

3.9.1 (also seeing this in 3.8.*)

Additional Context

Seems pretty likely that this commit introduced it, but there’s a lot going on so I’m not sure where:

https://github.com/tiangolo/fastapi/commit/97fa743ecb5a716fa0223a7463cb83f3f0df494c#diff-4939896f79697822446463d5d6dfbf7a0ea5805c46565d355c7305d86f3405c7

[fakegermano]

after another bit of search, I think this issue is a duplicate of both https://github.com/tiangolo/fastapi/issues/3665 and https://github.com/tiangolo/fastapi/issues/3782, there is also some more info there!

[trevorwang]

it’s http://localhost:8000/docs

It is not working now. I got the same error as yours. Sad.