Path parameter became required if it defined via Pydantic model

[gtors]

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Example Code

from pydantic import BaseModel, Field
from fastapi import Path, FastAPI, Depends

app = FastAPI()

class Foo(BaseModel):
    bar: str = Field(Path('bar'))  # default value is defined

@app.get('/foo')  # always will return 422 because `bar` not represent in path
async def pathless_handler(params: Foo = Depends(Foo)):     
    pass

@app.get('/foo/{bar}')
async def path_aware_handler(params: Foo = Depends(Foo)):
    pass

Description

If the path parameters are defined via Pydantic model, then the path parameters become always required.

curl -v localhost:8000/foo
*   Trying 127.0.0.1:8000...
* Connected to localhost (127.0.0.1) port 8000 (#0)
> GET /foo HTTP/1.1
> Host: localhost:8000
> User-Agent: curl/7.83.1
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 422 Unprocessable Entity
< date: Fri, 10 Jun 2022 11:56:49 GMT
< server: uvicorn
< content-length: 87
< content-type: application/json
<
* Connection #0 to host localhost left intact
{"detail":[{"loc":["path","bar"],"msg":"field required","type":"value_error.missing"}]}% 

The root of evil is in this line (it makes a path parameter always required, even if a default value is specified 😢)

fastapi/fastapi/params.py

Line 85 in 1876ebc

default=...,

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.78.0

Python Version

3.10.4

Additional Context

No response

[gtors]

As ad-hoc solution:

from typing import Any
from fastapi.params import Path as OrigPath, Undefined

class Path(OrigPath):

    def __init__(self, default: Any = Undefined, *args, **kwargs):
        super().__init__(*args, **kwargs)

        if default is not Undefined:
            self.default = default

[NickVeld]

I also have found that the following blocks setting of default.

fastapi/fastapi/params.py

Line 85 in 1876ebc

default=...,

If change it to default=default everything is fine.

I was wondering does it really have not been found before. But I have found this issue in the issue list.

Other related:

• Default values of Path parameters are missing in OpenAPI doc #4843
• Add default to Path parameter #4914
• Add default for default argument in Path #3370

[NickVeld]

@gtors @triangolo
As I see it is not a technical problem to implement it. It is not implemented because of OpenAPI.

Now there is a following situation:

• You are setting default
• After some layers (even not in the top-level) it is discarded using =...
• The developers does not understand anything and digging into the package code

My suggestion:

1. Remove default at all (but I understand that this parameter can be handy when OpenAPI will be changed)
2. Add a warning that signals that default will not be set.

I believe in both cases it is good to label the issue as a enchantment or a bug and not a question. Because nothing is asked but not intuitive behavior is shown.

I can create a pull request for both solutions.

[NickVeld]

As for the ad-hoc solution, in order to be consistent the code must redefine Path from fastapi.param_functions (the usual import is from fastapi import Path) as well:

from typing import Any, Dict, Optional

from fastapi.params import Path as OriginalParamsPath, Undefined


class ParamsPath(OriginalParamsPath):
    """Original ``Path`` in ``fastapi.params`` implementation discards ``default``

    See https://github.com/tiangolo/fastapi/issues/5019
    """
    def __init__(self, default: Any = Undefined, *args, **kwargs):
        super().__init__(*args, **kwargs)

        if default is not Undefined:
            self.default = default


def Path(  # noqa: N802
    default: Any = Undefined,
    *,
    alias: Optional[str] = None,
    title: Optional[str] = None,
    description: Optional[str] = None,
    gt: Optional[float] = None,
    ge: Optional[float] = None,
    lt: Optional[float] = None,
    le: Optional[float] = None,
    min_length: Optional[int] = None,
    max_length: Optional[int] = None,
    regex: Optional[str] = None,
    example: Any = Undefined,
    examples: Optional[Dict[str, Any]] = None,
    deprecated: Optional[bool] = None,
    include_in_schema: bool = True,
    **extra: Any,
) -> Any:
    """Redefined version of Path from ``fastapi.param_functions``"""
    return ParamsPath(
        default=default,
        alias=alias,
        title=title,
        description=description,
        gt=gt,
        ge=ge,
        lt=lt,
        le=le,
        min_length=min_length,
        max_length=max_length,
        regex=regex,
        example=example,
        examples=examples,
        deprecated=deprecated,
        include_in_schema=include_in_schema,
        **extra,
    )

It is cleaner to place the whole snippet as-is into a separate module; and then import it from there.

[tiangolo]

Defining path parameters (or other parameters that don't come from the body) using Pydantic models is not really supported.

There will be a way to do that in a future release, but not yet. So, for now, your have to take that path parameter out of the Pydantic model.

[github-actions]

Assuming the original need was handled, this will be automatically closed now. But feel free to add more comments or create new issues or PRs.

[NickVeld]

Defining path parameters (or other parameters that don't come from the body) using Pydantic models is not really supported.

There will be a way to do that in a future release, but not yet. So, for now, your have to take that path parameter out of the Pydantic model.

@tiangolo , it can be not implemented but the behavior stays confusing anyway. I proposed two solutions how to deal with it but you commented none of them.

[tiangolo]

Ah, yeah @NickVeld , I was focusing on the original post. So, the thing is, I'm not sure I understand what would be the intention of the original question, what would be expected in that case. I'm not sure what would mean to have a default value for a path parameter. I think I'm missing the explanation of what was expected, and what was @gtors intending to achieve.

Now, if you have a specific use case that needs something related to this, please create a new issue, following the template, and please make sure you add what are you intending to solve and what behavior you would have expected.