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Extending OpenAPI

There are some cases where you might need to modify the generated OpenAPI schema.

In this section you will see how.

The normal process

The normal (default) process, is as follows.

A FastAPI application (instance) has an .openapi() method that is expected to return the OpenAPI schema.

As part of the application object creation, a path operation for /openapi.json (or for whatever you set your openapi_url) is registered.

It just returns a JSON response with the result of the application's .openapi() method.

By default, what the method .openapi() does is check the property .openapi_schema to see if it has contents and return them.

If it doesn't, it generates them using the utility function at fastapi.openapi.utils.get_openapi.

And that function get_openapi() receives as parameters:

  • title: The OpenAPI title, shown in the docs.
  • version: The version of your API, e.g. 2.5.0.
  • openapi_version: The version of the OpenAPI specification used. By default, the latest: 3.1.0.
  • summary: A short summary of the API.
  • description: The description of your API, this can include markdown and will be shown in the docs.
  • routes: A list of routes, these are each of the registered path operations. They are taken from app.routes.

Info

The parameter summary is available in OpenAPI 3.1.0 and above, supported by FastAPI 0.99.0 and above.

Overriding the defaults

Using the information above, you can use the same utility function to generate the OpenAPI schema and override each part that you need.

For example, let's add ReDoc's OpenAPI extension to include a custom logo.

Normal FastAPI

First, write all your FastAPI application as normally:

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

app = FastAPI()


@app.get("/items/")
async def read_items():
    return [{"name": "Foo"}]


def custom_openapi():
    if app.openapi_schema:
        return app.openapi_schema
    openapi_schema = get_openapi(
        title="Custom title",
        version="2.5.0",
        summary="This is a very custom OpenAPI schema",
        description="Here's a longer description of the custom **OpenAPI** schema",
        routes=app.routes,
    )
    openapi_schema["info"]["x-logo"] = {
        "url": "https://fastapi.tiangolo.com/img/logo-margin/logo-teal.png"
    }
    app.openapi_schema = openapi_schema
    return app.openapi_schema


app.openapi = custom_openapi

Generate the OpenAPI schema

Then, use the same utility function to generate the OpenAPI schema, inside a custom_openapi() function:

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

app = FastAPI()


@app.get("/items/")
async def read_items():
    return [{"name": "Foo"}]


def custom_openapi():
    if app.openapi_schema:
        return app.openapi_schema
    openapi_schema = get_openapi(
        title="Custom title",
        version="2.5.0",
        summary="This is a very custom OpenAPI schema",
        description="Here's a longer description of the custom **OpenAPI** schema",
        routes=app.routes,
    )
    openapi_schema["info"]["x-logo"] = {
        "url": "https://fastapi.tiangolo.com/img/logo-margin/logo-teal.png"
    }
    app.openapi_schema = openapi_schema
    return app.openapi_schema


app.openapi = custom_openapi

Modify the OpenAPI schema

Now you can add the ReDoc extension, adding a custom x-logo to the info "object" in the OpenAPI schema:

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

app = FastAPI()


@app.get("/items/")
async def read_items():
    return [{"name": "Foo"}]


def custom_openapi():
    if app.openapi_schema:
        return app.openapi_schema
    openapi_schema = get_openapi(
        title="Custom title",
        version="2.5.0",
        summary="This is a very custom OpenAPI schema",
        description="Here's a longer description of the custom **OpenAPI** schema",
        routes=app.routes,
    )
    openapi_schema["info"]["x-logo"] = {
        "url": "https://fastapi.tiangolo.com/img/logo-margin/logo-teal.png"
    }
    app.openapi_schema = openapi_schema
    return app.openapi_schema


app.openapi = custom_openapi

Cache the OpenAPI schema

You can use the property .openapi_schema as a "cache", to store your generated schema.

That way, your application won't have to generate the schema every time a user opens your API docs.

It will be generated only once, and then the same cached schema will be used for the next requests.

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

app = FastAPI()


@app.get("/items/")
async def read_items():
    return [{"name": "Foo"}]


def custom_openapi():
    if app.openapi_schema:
        return app.openapi_schema
    openapi_schema = get_openapi(
        title="Custom title",
        version="2.5.0",
        summary="This is a very custom OpenAPI schema",
        description="Here's a longer description of the custom **OpenAPI** schema",
        routes=app.routes,
    )
    openapi_schema["info"]["x-logo"] = {
        "url": "https://fastapi.tiangolo.com/img/logo-margin/logo-teal.png"
    }
    app.openapi_schema = openapi_schema
    return app.openapi_schema


app.openapi = custom_openapi

Override the method

Now you can replace the .openapi() method with your new function.

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

app = FastAPI()


@app.get("/items/")
async def read_items():
    return [{"name": "Foo"}]


def custom_openapi():
    if app.openapi_schema:
        return app.openapi_schema
    openapi_schema = get_openapi(
        title="Custom title",
        version="2.5.0",
        summary="This is a very custom OpenAPI schema",
        description="Here's a longer description of the custom **OpenAPI** schema",
        routes=app.routes,
    )
    openapi_schema["info"]["x-logo"] = {
        "url": "https://fastapi.tiangolo.com/img/logo-margin/logo-teal.png"
    }
    app.openapi_schema = openapi_schema
    return app.openapi_schema


app.openapi = custom_openapi

Check it

Once you go to http://127.0.0.1:8000/redoc you will see that you are using your custom logo (in this example, FastAPI's logo):