OpenAPI schema customization in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the effect of setting openapi_url=None in FastAPI?

Consider this FastAPI app initialization:

app = FastAPI(openapi_url=None)

What will happen when you try to access /openapi.json?

FastAPI

from fastapi import FastAPI
app = FastAPI(openapi_url=None)

A The app will raise an error on startup because openapi_url cannot be None.

B The OpenAPI JSON schema will be available at /openapi.json as usual.

C The OpenAPI JSON schema will be available but at a different URL automatically.

D The OpenAPI JSON schema will not be available; accessing /openapi.json returns 404.

💡 Hint

Think about what disabling the OpenAPI schema URL does to the app's documentation endpoints.

Setting openapi_url=None disables the automatic generation of the OpenAPI JSON schema endpoint, so /openapi.json returns 404.

📝 Syntax

intermediate

Which code snippet correctly adds a custom OpenAPI schema title in FastAPI?

You want to customize the OpenAPI schema title to "My API". Which snippet does this correctly?

A app = FastAPI(title="My API")

B app = FastAPI(openapi_title="My API")

C app = FastAPI(openapi_schema_title="My API")

D app = FastAPI(schema_title="My API")

💡 Hint

Check the FastAPI constructor parameters for setting the API title.

The correct parameter to set the OpenAPI schema title is 'title'.

🔧 Debug

advanced

Why does this custom OpenAPI schema function cause a recursion error?

Given this code snippet:

def custom_openapi():
    return app.openapi()

app.openapi = custom_openapi

What causes the recursion error when calling app.openapi()?

FastAPI

def custom_openapi():
    return app.openapi()

app.openapi = custom_openapi

A The app.openapi attribute is not callable, causing a TypeError.

B The custom_openapi calls itself recursively without a base case.

C The function is missing a return statement.

D The app object is not defined in the function scope.

💡 Hint

Think about what happens when custom_openapi calls app.openapi after reassigning it.

Assigning app.openapi = custom_openapi means calling app.openapi() inside custom_openapi calls itself again, causing infinite recursion.

🧠 Conceptual

advanced

How can you add a custom field to the OpenAPI schema info section in FastAPI?

You want to add a custom field x-logo with a URL inside the OpenAPI info section. Which approach is correct?

A Add a route that returns the custom OpenAPI JSON with the extra field.

B Pass x-logo as a parameter to FastAPI constructor.

C Override the app.openapi() method to modify the schema dictionary before returning it.

D Use a middleware to inject the x-logo field into the response headers.

💡 Hint

Think about how to customize the OpenAPI JSON output directly.

Overriding app.openapi() lets you modify the generated OpenAPI schema dictionary, adding custom fields like x-logo.

❓ state_output

expert

What is the output of this FastAPI OpenAPI customization code?

Given this code snippet:

from fastapi import FastAPI

app = FastAPI()

original_openapi = app.openapi

def custom_openapi():
    if not hasattr(app, "_custom_openapi_schema"):
        schema = original_openapi()
        schema["info"]["title"] = "Custom API"
        app._custom_openapi_schema = schema
    return app._custom_openapi_schema

app.openapi = custom_openapi

print(app.openapi()["info"]["title"])

What will be printed?

FastAPI

from fastapi import FastAPI

app = FastAPI()

original_openapi = app.openapi

def custom_openapi():
    if not hasattr(app, "_custom_openapi_schema"):
        schema = original_openapi()
        schema["info"]["title"] = "Custom API"
        app._custom_openapi_schema = schema
    return app._custom_openapi_schema

app.openapi = custom_openapi

print(app.openapi()["info"]["title"])

A Custom API

B FastAPI

C None

D Raises AttributeError

💡 Hint

Look at how the title is changed and cached in the custom_openapi function.

The custom_openapi function modifies the title to 'Custom API' once and caches it, so the printed title is 'Custom API'.