Unconsistant use of Enum and const when dealing with literals

[LucasLev]

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• I already checked if it is not related to FastAPI but to Swagger UI.
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Example Code

from fastapi import FastAPI
from typing import Literal

app = FastAPI()


@app.get("/regular")
def i_want_a_fruit(fruit: Literal["apple", "peach"]):
    return fruit


@app.get("/tropical")
def i_want_a_tropical_fruit(tropical_fruit: Literal["pineapple"]):
    return tropical_fruit

Description

• Open the browser to the swagger docs
• navigate to the tropical endpoint

Expected :

• Drop down menu to select the tropical fruit (as in the /regular endpoint)

Current :

• Regular string field with no hint as to what the possible values are
  

Operating System

Linux, Windows, macOS

Operating System Details

No response

FastAPI Version

0.100.1

Python Version

3.11.3

Additional Context

No response

[LucasLev]

We can see from the openapi.json

        "/tropical": {
            "get": {
                "summary": "I Want A Tropical Fruit",
                "operationId": "i_want_a_tropical_fruit_tropical_get",
                "parameters": [
                    {
                        "name": "tropical_fruit",
                        "in": "query",
                        "required": true,
                        "schema": {
                            "const": "pineapple",
                            "title": "Tropical Fruit"
                        }
                    }
                ],
                "responses": {
                    "200": {
                        "description": "Successful Response",
                        "content": {
                            "application/json": {
                                "schema": {}
                            }
                        }
                    },
                    "422": {
                        "description": "Validation Error",
                        "content": {
                            "application/json": {
                                "schema": {
                                    "$ref": "#/components/schemas/HTTPValidationError"
                                }
                            }
                        }
                    }
                }
            }
        }

That a literal of length one is converted to a const. I think they should still remain a swagger enum.

[nikstuckenbrock]

I've got the same problem here. The main problem about the const is that it is not supported by OpenAPI by now. I'm not that deep into the sourcecode of FastAPI and Pydantic, but I would assume that FastAPI uses Pydantic to generate a json_schema for the OpenAPI specification. As mentioned in the Pydantic documentation they use the standard json_schema which supports const.

[sh-at-cs]

@nikstuckenbrock Just FYI: That documentation is for OpenAPI 3.0. const is supported in OpenAPI 3.1.

[marcelkwaschny]

Here is a little workaround to recursively replace all occurences of the const keyword in the dict of the generated openapi spec with the enum keyword:

import copy
import json

from fastapi import FastAPI
from fastapi.openapi.utils import get_openapi


api = FastAPI()


def walk_and_replace(node):
    for item in node.values():
        if isinstance(item, dict):
            if "const" in item:
                item["enum"] = [item["const"]]
                item["type"] = "string"
                del item["const"]
            walk_and_replace(item)


def custom_openapi():
    if api.openapi_schema:
        return api.openapi_schema

    openapi_schema = get_openapi(
        title=api.title,
        version=api.version,
        openapi_version="3.0.2",
        description=api.description,
        routes=api.routes,
    )

    walk_and_replace(openapi_schema)

    api.openapi_schema = openapi_schema
    return api.openapi_schema


api.openapi = custom_openapi