First Check

• I added a very descriptive title to this issue.
• I used the GitHub search to find a similar issue and didn't find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google "How to X in FastAPI" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Commit to Help

• I commit to help with one of those options 👆

Example Code

import json
from pydantic import BaseModel, Field
from fastapi import FastAPI
from fastapi import __version__ as fastapi_version

class Model(BaseModel):
    field: str = Field("foo", const=True)


app = FastAPI(title=f"Created with FastAPI v{fastapi_version}")


@app.get("/", response_model=Model)
def root():
    return Model(field="foo")

print(json.dumps(Model.schema(), indent=2))
print(json.dumps(app.openapi(), indent=2))

Description

With fastapi==0.68.0 and pydantic==1.8.2, the above MWE produces an invalid OpenAPI schema due to the handling of extra keys introduced in the last release. The inclusion of the const key is not supported in OpenAPI 3 (see Supported Keywords).

Here is a diff between the schema created for v0.65.2 vs v0.68 (also tested with 0.66, 0.67 with the same results).

// ...
  },
  "components": {
    "schemas": {
      "Model": {
        "title": "Model",
        "type": "object",
        "properties": {
          "field": {
            "title": "Field",
            "type": "string",
+           "const": "foo"
          }
// ...

This regression is caused by the addition of extra = "allow" to fastapi.openapi.models.Schema in (#1429) (see also the comment #1429 (comment)). This is something we ran into when updating FastAPI at Materials-Consortia/optimade-python-tools#887.

While I would also like to be able to use extension keys (with the appropriate x- prefix) too, this change now makes it hard to use some standard pydantic features. My suggestion would be to enumerate the keys from JSON schema that OpenAPI does not support (linked above) and handle those separately in the Config for Schema. Perhaps any "extra" key that is not in this list could be prepended with a x-my-app prefix defined on the init of the FastAPI instance. I'm happy to have a go at implementing this, if desired.

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.68.0

Python Version

3.8.5

Additional Context

I've made two gists for the OpenAPI schemas from v0.68.0 and v0.65.2:

• 0.65.2: https://gist.github.com/ml-evs/99b33f68b74a0c197e970f9222faa2bc
• 0.68.0: https://gist.github.com/ml-evs/824f9ac510f92d4e180a4e984d1da38d

You can pass the raw json through the online swagger validator to verify:

• 0.65.2: https://validator.swagger.io/validator/debug?url=https://gist.githubusercontent.com/ml-evs/99b33f68b74a0c197e970f9222faa2bc/raw/71d056de37a942bb9bc26943151c477a97aa18f4/fastapi_v0.65.2_openapi.json

  $ curl -s https://validator.swagger.io/validator/debug\?url\=https://gist.githubusercontent.com/ml-evs/99b33f68b74a0c197e970f9222faa2bc/raw/71d056de37a942bb9bc26943151c477a97aa18f4/fastapi_v0.65.2_openapi.json | jq
  {}

• 0.68.0: https://validator.swagger.io/validator/debug?url=https://gist.githubusercontent.com/ml-evs/824f9ac510f92d4e180a4e984d1da38d/raw/d25b9c3a7802702ced6e3f7a1aa60e017ac9b339/fastapi_v0.68.0_openapi.json

  $ curl -s https://validator.swagger.io/validator/debug\?url\=https://gist.githubusercontent.com/ml-evs/824f9ac510f92d4e180a4e984d1da38d/raw/d25b9c3a7802702ced6e3f7a1aa60e017ac9b339/fastapi_v0.68.0_openapi.json | jq
  {
    "messages": [
      "attribute components.schemas.Model.const is unexpected"
    ],
    "schemaValidationMessages": [
      {
        "level": "error",
        "domain": "validation",
        "keyword": "oneOf",
        "message": "instance failed to match exactly one schema (matched 0 out of 2)",
        "schema": {
          "loadingURI": "#",
          "pointer": "/definitions/Components/properties/schemas/patternProperties/^[a-zA-Z0-9\\.\\-_]+$"
        },
        "instance": {
          "pointer": "/components/schemas/Model"
        }
      }
    ]
  }