[QUESTION] How do I use a string-formatted BaseModel as a path param?

Example

from fastapi import FastAPI
from pydantic import BaseModel

class Multihash(BaseModel):
    func: int
    digest: bytes

class MultihashHex(Multihash):
    @classmethod
    def __modify_schema__(cls, field_schema: Dict[str, Any]) -> None:
        field_schema.update({"type": "string", "format": "multihash-hex"})

    @classmethod
    def __get_validators__(cls) -> Iterator[Callable[..., Any]]:
        yield cls.validate

    @classmethod
    def validate(cls, value: Union[str, bytes]) -> Multihash:
        if isinstance(value, str):
            value = bytes.fromhex(value)
        return Multihash(...)


app = FastAPI()

@app.get("/data/{mh}")
def read(mh:MultihashHex):
    return ...

Description

When I run the above, I get:

$ uvicorn test:app
...
  File "./test.py", line 32, in <module>
    def read(mh:MultihashHex):
  File "/home/amnesia/Persistent/venv/lib/python3.7/site-packages/fastapi/routing.py", line 574, in decorator
    callbacks=callbacks,
  File "/home/amnesia/Persistent/venv/lib/python3.7/site-packages/fastapi/routing.py", line 520, in add_api_route
    callbacks=current_callbacks,
  File "/home/amnesia/Persistent/venv/lib/python3.7/site-packages/fastapi/routing.py", line 380, in __init__
    self.dependant = get_dependant(path=self.path_format, call=self.endpoint)
  File "/home/amnesia/Persistent/venv/lib/python3.7/site-packages/fastapi/dependencies/utils.py", line 308, in get_dependant
    ), "Path params must be of one of the supported types"
AssertionError: Path params must be of one of the supported types

I’m building an app that vends content-addressable data. I’d like to use a multihash as a path param. Multihash is just a (hash-function, hash-digest) tuple, with well-defined encoding.

My intent is to have my path param be a hexadecimal multihash, and use the Multihash type internally. There seem to be other examples of string-representable objects, like NameEmail or IPv4Address.

But, I can’t seem to make my Multihash class be a BaseModel. I get the above error.

I can make Multihash a base object instead, but I lose pydantic features.

Is it an error that BaseModel objects can’t be used this way, or is there a better way to do what I’m doing?

Environment

• OS: tails 4.17
• python: 3.7.3
• packages:

fastapi==0.63.0
pydantic==1.8.1

[AllSeeingEyeTolledEweSew]

as you can see here in the docs: https://fastapi.tiangolo.com/tutorial/body/#request-body-path-query-parameters

Reading that:

If the parameter is also declared in the path, it will be used as a path parameter. If the parameter is of a singular type (like int, float, str, bool, etc) it will be interpreted as a query parameter. If the parameter is declared to be of the type of a Pydantic model, it will be interpreted as a request body.

Reading it in order, I thought it says plainly that if a parameter is declared in the path then it’s used as a path parameter, even if it’s a BaseModel.

If it intends to say that “BaseModels will always be interpreted as request body”, then… I feel it should be written differently 😃

I know is_scalar_field is part of the stack that blocks this usage. But surely an undocumented utility function is not ground truth for APIs or intent

Extending my example from above:

class Outer(BaseModel):
    mh: MultihashHex

parse_raw_as(Outer, '{"mh": "0a2b3c..."}')
"""
Outer(mh=MultihashHex(...))
"""

Pydantic lets me parse a string field into a BaseModel just fine, so I expect to be able to parse a path parameter into a BaseModel as well. The two seem perfectly analogous to me

[g0di]

Came to the exact same issue when I wanted to provide a path parameter which is an object with several keys. As mentioned previously, a test in source code prevent us from being able to provide something else than scalars as path parameters…

However, the OpenAPI specification currently allow and define a way to document path parameters as objects or arrays thanks to schema, style and explode properties of path parameter objects, see https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.0.md#parameter-object

IMO, It could be interesting to implement such a thing