Why middleware processes requests globally in FastAPI - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a middleware function in FastAPI.

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.middleware("[1]")
async def log_requests(request, call_next):
    response = await call_next(request)
    return response

Drag options to blanks, or click blank then click option'

Arequest

Bhttp

Cmiddleware

Droute

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'request' or 'route' instead of 'http' in the decorator.

Forgetting to make the middleware function async.

The middleware decorator in FastAPI uses the string "http" to indicate it processes HTTP requests globally.

2fill in blank

medium

Complete the code to call the next middleware or route handler inside the middleware.

FastAPI

async def log_requests(request, call_next):
    response = await [1](request)
    return response

Drag options to blanks, or click blank then click option'

Arequest

Bhandle_request

Cnext_call

Dcall_next

Attempts:

3 left

💡 Hint

Common Mistakes

Calling 'request' or other variables instead of 'call_next'.

Not using await before call_next.

Inside middleware, you call the next handler using the 'call_next' function with the request.

3fill in blank

hard

Fix the error in the middleware function to ensure it processes all requests globally.

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.middleware("http")
async def [1](request, call_next):
    response = await call_next(request)
    return response

Drag options to blanks, or click blank then click option'

Alog_requests

Bprocess_request

Chandle_request

Dmiddleware_func

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic or unclear function names.

Using reserved keywords as function names.

The middleware function name can be anything, but 'log_requests' is the conventional name used here to indicate its purpose.

4fill in blank

hard

Fill both blanks to create a middleware that adds a custom header to every response.

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.middleware("http")
async def add_header(request, call_next):
    response = await call_next(request)
    response.headers[[1]] = [2]
    return response

Drag options to blanks, or click blank then click option'

A"X-Custom-Header"

B"Content-Type"

C"Hello"

D"application/json"

Attempts:

3 left

💡 Hint

Common Mistakes

Using standard headers like 'Content-Type' instead of a custom one.

Not using quotes around header names and values.

The middleware adds a custom header named "X-Custom-Header" with the value "Hello" to every response.

5fill in blank

hard

Fill both blanks to create a middleware that logs the request method and path before processing.

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.middleware("http")
async def log_request_info(request, call_next):
    print(f"Request: [1] [2]")
    response = await call_next(request)
    return response

Drag options to blanks, or click blank then click option'

Arequest.method

Brequest.url.path

Crequest.headers

Drequest.body

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'request.headers' or 'request.body' instead of method and path for logging.

Not using f-string formatting correctly.

The middleware prints the HTTP method and URL path of the incoming request before calling the next handler.

Practice

1. Why does FastAPI middleware process requests globally across all routes?

easy

A. To run only on specific routes chosen by the developer

B. To handle shared tasks like logging or authentication once for all requests

C. To replace route handlers completely

D. To slow down the application for debugging

Solution

1. Step 1: Understand middleware purpose

   Middleware is designed to run code before and after every request to handle common tasks.

2. Step 2: Recognize global effect

   It applies globally to avoid repeating the same code in each route handler.

3. Final Answer:

   To handle shared tasks like logging or authentication once for all requests -> Option B

4. Quick Check:

   Middleware = shared tasks globally [OK]

Hint: Middleware runs for all requests to avoid code repetition [OK]

Common Mistakes:

• Thinking middleware runs only on selected routes
• Believing middleware replaces route handlers
• Assuming middleware slows down app intentionally

2. Which of the following is the correct way to add middleware globally in FastAPI?

easy

A. app.add_middleware(SomeMiddleware)

B. app.middleware(SomeMiddleware)

C. app.route.middleware(SomeMiddleware)

D. SomeMiddleware(app)

Solution

1. Step 1: Recall FastAPI middleware syntax

   FastAPI uses the method add_middleware() on the app instance to add middleware globally.

2. Step 2: Eliminate incorrect options

   Options A, B and C are invalid: A instantiates middleware without adding it to the app, B and C are invalid method calls.

3. Final Answer:

   app.add_middleware(SomeMiddleware) -> Option A

4. Quick Check:

   Use add_middleware() to add middleware globally [OK]

Hint: Use app.add_middleware() to add middleware globally [OK]

Common Mistakes:

• Using app.middleware() which does not exist
• Trying to add middleware on route instead of app
• Instantiating middleware without adding it to app

3. Given this middleware code in FastAPI:

from fastapi import FastAPI
from starlette.middleware.base import BaseHTTPMiddleware

class PrintMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        print("Before request")
        response = await call_next(request)
        print("After request")
        return response

app = FastAPI()
app.add_middleware(PrintMiddleware)

@app.get("/hello")
async def hello():
    return {"message": "Hello"}

What will be printed when a client requests /hello?

medium

A. Before request
After request

B. After request
Before request

C. Only Before request

D. No output printed

Solution

1. Step 1: Understand middleware dispatch flow

   The middleware prints "Before request" before calling the next handler, then "After request" after the response is received.

2. Step 2: Trace the request lifecycle

   When /hello is requested, the middleware prints "Before request", then the route runs, then prints "After request".

3. Final Answer:

   Before request After request -> Option A

4. Quick Check:

   Middleware prints before and after request [OK]

Hint: Middleware prints before and after call_next() [OK]

Common Mistakes:

• Assuming prints happen in reverse order
• Thinking only one print runs
• Believing middleware does not print anything

4. You wrote this middleware but it does not run for any requests:

class MyMiddleware:
    async def dispatch(self, request, call_next):
        print("Middleware active")
        response = await call_next(request)
        return response

app = FastAPI()
app.add_middleware(MyMiddleware)

What is the likely problem?

medium

A. Middleware must be added after route definitions

B. dispatch method should be synchronous

C. MyMiddleware does not inherit from BaseHTTPMiddleware

D. Middleware class must be decorated with @middleware

Solution

1. Step 1: Check middleware class inheritance

   FastAPI middleware classes must inherit from BaseHTTPMiddleware or implement ASGI interface properly.

2. Step 2: Identify missing inheritance

   MyMiddleware lacks inheritance, so FastAPI cannot use it as middleware.

3. Final Answer:

   MyMiddleware does not inherit from BaseHTTPMiddleware -> Option C

4. Quick Check:

   Middleware must inherit BaseHTTPMiddleware [OK]

Hint: Middleware class must inherit BaseHTTPMiddleware [OK]

Common Mistakes:

• Making dispatch synchronous instead of async
• Adding middleware before routes (order usually doesn't block)
• Thinking @middleware decorator is required for class middleware

5. You want to add middleware that logs request time but only for routes under /api. Why does FastAPI middleware still run on all routes, and how can you limit it?

hard

A. FastAPI does not support middleware; use dependencies instead

B. Middleware can be added only to specific routes by passing route list to add_middleware

C. Middleware runs globally but can be disabled per route with a decorator

D. Middleware runs globally by design; to limit, check path inside middleware and skip non-/api requests

Solution

1. Step 1: Understand middleware scope

   FastAPI middleware always runs globally on every request to handle shared tasks.

2. Step 2: Limit middleware effect by path check

   To restrict middleware to /api routes, check the request URL path inside middleware and skip processing for others.

3. Final Answer:

   Middleware runs globally by design; to limit, check path inside middleware and skip non-/api requests -> Option D

4. Quick Check:

   Middleware global; filter inside middleware [OK]

Hint: Middleware always global; filter requests inside middleware [OK]

Common Mistakes:

• Expecting middleware to attach only to some routes automatically
• Trying to pass routes to add_middleware (not supported)
• Thinking middleware can be disabled per route with decorators