FastAPI error when using Annotated in Class dependencies

Question

FastAPI added support for Annotated (and started recommending it) in version 0.95.0.

Additionally, FastAPI has a very powerful but intuitive Dependency Injection system (documentation). Moreover, FastAPI support Classes as Dependencies.

However, it seams like Annotated cannot be used in class dependencies, but on function dependencies.

I use FastAPI version 0.110.1.

from __future__ import annotations

from typing import Annotated

from fastapi import FastAPI, Depends, Query

app = FastAPI()


class ClassDependency:
    def __init__(self, name: Annotated[str, Query(description="foo")]):
        self.name = name

async def function_dependency(name: Annotated[str, Query(description="foo")]) -> dict:
    return {"name": name}


@app.get("/")
async def search(c: Annotated[ClassDependency, Depends(function_dependency)]) -> dict:
    return {"c": c.name}

The example above works without errors, but if I replace Depends(function_dependency) with Depends(ClassDependency) an exception is raised with the following message:

pydantic.errors.PydanticUndefinedAnnotation: name 'Query' is not defined

Then, if I remove Annotated from the ClassDependency, by replacing the name: Annotated[str, Query(description="foo")] with the name: str, the example works.

My question: Can I use Class dependencies and put Annotated to the parameters set in the constructor? Because it seams this is not working.

My need: I want to have a class hierarchy for the query params of my api endpoints and provide validation and documentation extras for each of the param.

Answer 1

Remove the following and it will work.

from __future__ import annotations