Automatic API documentation (Swagger UI) in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the default URL path for Swagger UI in FastAPI?

FastAPI automatically generates API documentation using Swagger UI. By default, where can you access this interactive documentation in a FastAPI app?

A /documentation

B /docs

C /swagger

D /api-docs

💡 Hint

Think about the common short path FastAPI uses for docs.

FastAPI serves Swagger UI at the path '/docs' by default, allowing easy access to interactive API docs.

📝 Syntax

intermediate

Which code snippet correctly disables Swagger UI in FastAPI?

You want to disable the automatic Swagger UI documentation in your FastAPI app. Which of the following code snippets achieves this?

A app = FastAPI(docs_url=None)

B app = FastAPI(swagger_ui=False)

C app = FastAPI(enable_swagger=False)

D app = FastAPI(swagger=False)

💡 Hint

Check the official FastAPI parameter names for disabling docs.

Setting docs_url=None disables the Swagger UI docs in FastAPI. Other parameters are invalid.

❓ state_output

advanced

What is the output of accessing /docs after setting openapi_url=None?

Consider this FastAPI app code: app = FastAPI(openapi_url=None) What happens when you visit the /docs URL in a browser?

FastAPI

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

A A 404 Not Found error page

B The Swagger UI page loads but shows no API endpoints

C The Swagger UI page loads normally with API endpoints

D A 500 Internal Server Error

💡 Hint

Swagger UI depends on the OpenAPI schema URL to load.

Disabling openapi_url removes the OpenAPI schema endpoint, so Swagger UI cannot load and /docs returns 404.

🧠 Conceptual

advanced

Why does FastAPI generate API docs automatically?

FastAPI automatically creates interactive API documentation using Swagger UI. What is the main reason this works without extra code?

A FastAPI uses decorators to generate HTML pages for docs

B FastAPI reads docstrings from each function to build docs

C FastAPI uses Python type hints to generate OpenAPI schema automatically

D FastAPI requires manual JSON schema files to generate docs

💡 Hint

Think about how FastAPI knows the data types and endpoints.

FastAPI leverages Python type hints and function signatures to build the OpenAPI schema automatically, enabling docs generation.

🔧 Debug

expert

Why does Swagger UI show empty API list despite endpoints defined?

You wrote this FastAPI app: from fastapi import FastAPI app = FastAPI(docs_url='/docs') @app.get('/items') async def read_items(): return [{'item_id': 1}] But when you open /docs, the API list is empty. What is the most likely cause?

FastAPI

from fastapi import FastAPI
app = FastAPI(docs_url='/docs')

@app.get('/items')
async def read_items():
    return [{'item_id': 1}]

A The docs_url path conflicts with another route

B The route decorator @app.get is missing parentheses

C The function read_items is not async

D The openapi_url parameter was set to None elsewhere, disabling schema generation

💡 Hint

Swagger UI depends on the OpenAPI schema endpoint to list APIs.

If openapi_url is None, the OpenAPI schema is disabled, so Swagger UI shows no endpoints even if routes exist.

Practice

1. What is the default URL path where FastAPI serves the automatic API documentation using Swagger UI?

easy

A. /docs

B. /swagger

C. /api-docs

D. /documentation

Solution

1. Step 1: Understand FastAPI default docs path

   FastAPI automatically provides Swagger UI documentation at the path '/docs' by default.

2. Step 2: Compare options with default path

   Among the options, only '/docs' matches the default URL for Swagger UI in FastAPI.

3. Final Answer:

   /docs -> Option A

4. Quick Check:

   Default Swagger UI docs path = /docs [OK]

Hint: Remember: FastAPI docs are at /docs by default [OK]

Common Mistakes:

• Confusing /swagger with /docs
• Assuming /api-docs is default
• Thinking documentation is at root /

2. Which of the following code snippets correctly imports and creates a FastAPI app that will have automatic Swagger UI documentation?

easy

A. import fastapi
app = fastapi.App()

B. from fastapi import API
app = API()

C. from fastapi import FastAPI
app = FastAPI()

D. import FastAPI
app = FastAPI()

Solution

1. Step 1: Check correct import and app creation

   The correct import is 'from fastapi import FastAPI' and app is created by calling FastAPI().

2. Step 2: Validate each option

   from fastapi import FastAPI app = FastAPI() matches the correct syntax. Options A, C, and D have wrong imports or class names.

3. Final Answer:

   from fastapi import FastAPI\napp = FastAPI() -> Option C

4. Quick Check:

   Correct import and app creation = from fastapi import FastAPI app = FastAPI() [OK]

Hint: FastAPI app is created by FastAPI() after correct import [OK]

Common Mistakes:

• Using wrong import names like API or App
• Trying to import FastAPI with capital letter as module
• Calling wrong constructor names

3. Given this FastAPI code snippet, what will you see when you open http://localhost:8000/docs in your browser?

from fastapi import FastAPI
app = FastAPI()

@app.get("/hello")
def say_hello():
    return {"message": "Hello, world!"}

medium

A. A 404 Not Found error because /docs is not defined

B. A plain JSON response with {"message": "Hello, world!"}

C. A blank page with no content

D. An interactive Swagger UI showing the /hello GET endpoint with a Try it out button

Solution

1. Step 1: Understand FastAPI automatic docs behavior

   FastAPI automatically generates Swagger UI at /docs showing all defined endpoints.

2. Step 2: Analyze the given code

   The code defines a GET endpoint /hello. The docs page will list this endpoint with interactive testing.

3. Final Answer:

   An interactive Swagger UI showing the /hello GET endpoint with a Try it out button -> Option D

4. Quick Check:

   FastAPI auto docs show endpoints interactively at /docs [OK]

Hint: Open /docs to see interactive API docs with all endpoints [OK]

Common Mistakes:

• Expecting JSON response at /docs instead of UI
• Thinking /docs must be manually coded
• Confusing /docs with endpoint paths

4. You wrote this FastAPI app but when you visit /docs, you get a 404 error. What is the likely cause?

from fastapi import FastAPI
app = FastAPI()

@app.get("/items")
def read_items():
    return [{"item_id": 1}]

# You run the app with: uvicorn main:app --port 8000 --host 0.0.0.0

# But /docs returns 404

medium

A. You are running the app with the wrong module or app name

B. The endpoint /items is missing

C. You need to add extra code to enable /docs

D. You forgot to import FastAPI

Solution

1. Step 1: Check the run command and app reference

   The command 'uvicorn main:app' requires a file named main.py with an app variable.

2. Step 2: Identify mismatch causing 404 on /docs

   If the file or app name is wrong, uvicorn won't serve the correct app, so /docs is missing.

3. Final Answer:

   You are running the app with the wrong module or app name -> Option A

4. Quick Check:

   Run command must match file and app names exactly [OK]

Hint: Match uvicorn run command module:app exactly [OK]

Common Mistakes:

• Assuming /docs needs manual enabling
• Forgetting to import FastAPI (but code shows import)
• Thinking missing endpoint causes /docs 404

5. You want to customize the automatic API docs path from the default /docs to /api-docs. Which FastAPI app creation code achieves this?

hard

A. app = FastAPI(swagger_ui_url="/api-docs")

B. app = FastAPI(docs_url="/api-docs")

C. app = FastAPI(openapi_url="/api-docs")

D. app = FastAPI(docs_path="/api-docs")

Solution

1. Step 1: Identify FastAPI parameter for docs path

   The parameter to change Swagger UI docs path is docs_url.

2. Step 2: Match option with correct parameter

   app = FastAPI(docs_url="/api-docs") uses docs_url="/api-docs", which correctly changes the docs path.

3. Final Answer:

   app = FastAPI(docs_url="/api-docs") -> Option B

4. Quick Check:

   Change docs path with docs_url parameter [OK]

Hint: Use docs_url to set custom docs path [OK]

Common Mistakes:

• Using swagger_ui_url which changes UI assets path
• Changing openapi_url affects JSON schema, not docs UI
• Using docs_path which is not a valid parameter