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Example Code

from enum import Enum
from fastapi import FastAPI, Depends


class ModelName(str, Enum):
    alexnet = "alexnet"
    resnet = "resnet"
    lenet = "lenet"


def testfunc(model_name: ModelName): # <--- Type hinting here changes behaviour
    return model_name+"123"

app = FastAPI()

@app.get("/models/{model_name}")
async def get_model(model_name: ModelName, test = Depends(testfunc)):
    if model_name == ModelName.alexnet:
        return {"model_name": model_name, "test": test, "message": "Deep Learning FTW!"}

    if model_name.value == "lenet":
        return {"model_name": model_name, "test": test, "message": "LeCNN all the images"}

    return {"model_name": model_name, "test": test, "message": "Have some residuals"}

Description

Hello,

I came across a strange behaviour while progamming an app.
I managed to reproduce it starting from this example code in the doc : https://fastapi.tiangolo.com/tutorial/path-params/#predefined-values

I've just added a dependancy that uses the parameter.

...
def testfunc(model_name):
    return model_name+"123"
  
...

@app.get("/models/{model_name}")
async def get_model(model_name: ModelName, test = Depends(testfunc)):
...

But once done, the type checking no longer worked for the model_name parameter.
In the doc, the field was no longer displayed as a select input but a standard text input.

I tried a few things to figure out and finally found that adding the type hint in the dependancy function solved the problem, ensuring validation and displaying a select field in the doc.

Is there an explanation to this ? Does the dependancy injection override the type checking by "catching" the parameter directly ?
I couldn't find any technical detail about this behaviour and I would like to be sure to understand it so that I'll be sure not misusing it.

Thank you

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.70.0

Python Version

3.9.5

Additional Context

No response