Trailing slash behavior in FastAPI - Mini Project: Build & Apply

Trailing slash behavior

📖 Scenario: You are building a simple web API using FastAPI. You want to understand how FastAPI handles URLs with and without trailing slashes.

🎯 Goal: Create two API endpoints: one with a trailing slash and one without. Observe how FastAPI responds to requests to these endpoints with and without trailing slashes.

📋 What You'll Learn

Create a FastAPI app instance named app

Create a GET endpoint at /items/ (with trailing slash) that returns a JSON message

Create a GET endpoint at /users (without trailing slash) that returns a JSON message

Use the exact function names read_items and read_users for the endpoints

💡 Why This Matters

🌍 Real World

Web APIs often have URLs with or without trailing slashes. Understanding how your framework handles these helps avoid broken links or unexpected redirects.

💼 Career

Backend developers must know how to define routes correctly and handle URL patterns to build reliable APIs.

1

Create FastAPI app instance

Import FastAPI from fastapi and create an app instance called app.

FastAPI

from fastapi import FastAPI

# Create FastAPI app instance
# Your code here

Hint

Use app = FastAPI() to create the app instance.

2

Add GET endpoint with trailing slash

Add a GET endpoint at /items/ (with trailing slash) using @app.get("/items/"). Define a function called read_items that returns {"message": "Items endpoint with slash"}.

FastAPI

from fastapi import FastAPI

app = FastAPI()

# Add GET endpoint at /items/ with trailing slash
# Your code here

Hint

Use @app.get("/items/") decorator and define read_items function.

3

Add GET endpoint without trailing slash

Add a GET endpoint at /users (without trailing slash) using @app.get("/users"). Define a function called read_users that returns {"message": "Users endpoint without slash"}.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/items/")
def read_items():
    return {"message": "Items endpoint with slash"}

# Add GET endpoint at /users without trailing slash
# Your code here

Hint

Use @app.get("/users") decorator and define read_users function.

4

Add root endpoint to test trailing slash behavior

Add a GET endpoint at / using @app.get("/"). Define a function called read_root that returns {"message": "Root endpoint"}. This helps test how FastAPI handles root URL with or without slash.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/items/")
def read_items():
    return {"message": "Items endpoint with slash"}

@app.get("/users")
def read_users():
    return {"message": "Users endpoint without slash"}

# Add GET endpoint at / (root)
# Your code here

Hint

Use @app.get("/") decorator and define read_root function.

Practice

1. In FastAPI, what happens if you define a route with a trailing slash like @app.get('/items/') and a client requests /items without the slash?

easy

A. FastAPI will redirect the client to /items/ automatically.

B. FastAPI will return a 404 Not Found error.

C. FastAPI will treat both URLs as the same without any redirect.

D. FastAPI will raise a server error.

Solution

1. Step 1: Understand FastAPI's default trailing slash behavior

   FastAPI automatically redirects requests without a trailing slash to the route with a trailing slash if defined that way.

2. Step 2: Apply this to the given route

   Since the route is defined with a trailing slash, a request to /items will redirect to /items/.

3. Final Answer:

   FastAPI will redirect the client to /items/ automatically. -> Option A

4. Quick Check:

   Trailing slash route causes redirect = C [OK]

Hint: Trailing slash routes redirect missing slash URLs [OK]

Common Mistakes:

• Thinking FastAPI treats slash and no-slash URLs identically without redirect
• Assuming 404 error on missing slash
• Believing server error occurs

2. Which of the following is the correct way to define a FastAPI route that matches exactly /users without a trailing slash?

easy

A. @app.get('/users/')

B. @app.get('/users')

C. @app.get('/users/*')

D. @app.get('/users?')

Solution

1. Step 1: Recall route syntax for exact path matching

   To match a path without trailing slash exactly, define the route without the slash.

2. Step 2: Check each option

   @app.get('/users') uses @app.get('/users') which matches /users exactly without trailing slash.

3. Final Answer:

   @app.get('/users') -> Option B

4. Quick Check:

   Exact no-slash route = D [OK]

Hint: No trailing slash in route means no slash in URL [OK]

Common Mistakes:

• Adding trailing slash when none is wanted
• Using wildcard or query syntax incorrectly
• Confusing optional trailing slash with exact match

3. Given this FastAPI code snippet:

@app.get('/products')
async def get_products():
    return {'message': 'No slash'}

@app.get('/products/')
async def get_products_slash():
    return {'message': 'With slash'}

What will be the JSON response when a client requests /products/?

medium

A. {'message': 'With slash'}

B. {'message': 'No slash'}

C. 404 Not Found error

D. Redirect to /products without slash

Solution

1. Step 1: Identify route matching for /products/

   The route @app.get('/products/') matches the URL /products/ exactly.

2. Step 2: Determine which function runs

   The function get_products_slash handles /products/ and returns {'message': 'With slash'}.

3. Final Answer:

   {'message': 'With slash'} -> Option A

4. Quick Check:

   Trailing slash URL matches trailing slash route = A [OK]

Hint: Trailing slash URL matches trailing slash route exactly [OK]

Common Mistakes:

• Assuming no slash route handles slash URL
• Expecting redirect instead of direct match
• Confusing 404 error with route mismatch

4. You have this FastAPI route:

@app.get('/orders')

But when you request /orders/ with a trailing slash, you get a 404 error. How can you fix this to accept both /orders and /orders/ without errors?

medium

A. Add a middleware to ignore trailing slashes

B. Change the route to @app.get('/orders/') only

C. Use a wildcard route like @app.get('/orders*')

D. Define a second route with trailing slash: @app.get('/orders/')

Solution

1. Step 1: Understand FastAPI's strict matching

   FastAPI treats /orders and /orders/ as different routes and does not match one to the other automatically unless redirect applies.

2. Step 2: Add explicit route for trailing slash

   Defining @app.get('/orders/') alongside @app.get('/orders') allows both URLs to be handled without 404.

3. Final Answer:

   Define a second route with trailing slash: @app.get('/orders/') -> Option D

4. Quick Check:

   Explicit routes for both slash and no-slash = A [OK]

Hint: Add both slash and no-slash routes to avoid 404 [OK]

Common Mistakes:

• Assuming one route handles both URLs automatically
• Using wildcard incorrectly
• Expecting middleware to fix trailing slash issues by default

5. You want a FastAPI route to accept both /profile and /profile/ URLs without redirect or 404 errors, and return the same response. Which approach is best?

hard

A. Define one route with trailing slash and rely on FastAPI redirect.

B. Define one route without trailing slash and rely on FastAPI redirect.

C. Define two routes: @app.get('/profile') and @app.get('/profile/') with the same function.

D. Use a custom middleware to strip trailing slashes before routing.

Solution

1. Step 1: Understand FastAPI's redirect behavior

   FastAPI redirects missing trailing slash URLs to the defined route with trailing slash, but this causes a redirect, not direct handling.

2. Step 2: Avoid redirects by defining both routes

   Defining both @app.get('/profile') and @app.get('/profile/') with the same function handles both URLs directly without redirect or 404.

3. Final Answer:

   Define two routes: @app.get('/profile') and @app.get('/profile/') with the same function. -> Option C

4. Quick Check:

   Two explicit routes avoid redirect and 404 = B [OK]

Hint: Define both slash and no-slash routes for no redirect [OK]

Common Mistakes:

• Relying on redirect when direct handling is needed
• Using middleware unnecessarily
• Defining only one route expecting both URLs to work