Path parameters in FastAPI - Mini Project: Build & Apply

Using Path Parameters in FastAPI

📖 Scenario: You are building a simple web API for a bookstore. You want to create endpoints that can receive book IDs directly in the URL path to fetch details about specific books.

🎯 Goal: Build a FastAPI app that uses path parameters to get book details by their ID.

📋 What You'll Learn

Create a FastAPI app instance

Define a path parameter called book_id in the route

Return a dictionary with the book_id in the response

💡 Why This Matters

🌍 Real World

APIs often need to get specific resources by ID or name directly from the URL path. This project shows how to do that with FastAPI.

💼 Career

Understanding path parameters is essential for backend developers building RESTful APIs with FastAPI or similar frameworks.

1

Create a FastAPI app instance

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

FastAPI

from fastapi import FastAPI

# Create the FastAPI app instance called app
# Your code here

Hint

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

2

Define a path parameter in the route

Create a GET route at /books/{book_id} using @app.get. Define a function called read_book that takes book_id as an integer parameter.

FastAPI

from fastapi import FastAPI

app = FastAPI()

# Create a GET route at /books/{book_id} with a function read_book(book_id: int)
# Your code here

Hint

Use @app.get("/books/{book_id}") and define read_book(book_id: int).

3

Return the book ID in the response

Inside the read_book function, return a dictionary with the key book_id and the value from the book_id parameter.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/books/{book_id}")
def read_book(book_id: int):
    # Return a dictionary with book_id
    # Your code here

Hint

Return {"book_id": book_id} to send the book ID back.

4

Add type hint and run the app

Make sure the book_id parameter in read_book has the type hint int. This helps FastAPI convert the path parameter to an integer automatically.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/books/{book_id}")
def read_book(book_id: int):
    return {"book_id": book_id}

# Ensure book_id has type int for automatic conversion
# Your code here

Hint

Type hint book_id: int is required for FastAPI to parse the path parameter as an integer.

Practice

1. What is the purpose of path parameters in FastAPI?

easy

A. To send data only in the request body

B. To define static routes without variables

C. To capture values directly from the URL path

D. To handle only query string parameters

Solution

1. Step 1: Understand what path parameters do

   Path parameters allow the API to accept dynamic values from the URL path itself, making routes flexible.

2. Step 2: Compare with other data sources

   Unlike query parameters or request bodies, path parameters are part of the URL path, not optional or in the body.

3. Final Answer:

   To capture values directly from the URL path -> Option C

4. Quick Check:

   Path parameters = URL path values [OK]

Hint: Path parameters are always inside curly braces {} in the URL [OK]

Common Mistakes:

• Confusing path parameters with query parameters
• Thinking path parameters come from request body
• Assuming path parameters are optional by default

2. Which of the following is the correct way to declare a path parameter named item_id in FastAPI?

easy

A. @app.get("/items/:item_id")

B. @app.get("/items/item_id")

C. @app.get("/items/<item_id>")

D. @app.get("/items/{item_id}")

Solution

1. Step 1: Recall FastAPI path parameter syntax

   FastAPI uses curly braces {} to declare path parameters inside route strings.

2. Step 2: Identify the correct syntax

   Only @app.get("/items/{item_id}") uses curly braces correctly: "/items/{item_id}".

3. Final Answer:

   @app.get("/items/{item_id}") -> Option D

4. Quick Check:

   Curly braces {} = path parameters [OK]

Hint: Use curly braces {} around parameter names in route paths [OK]

Common Mistakes:

• Using quotes or angle brackets instead of curly braces
• Forgetting to wrap parameter name in braces
• Using colon syntax like other frameworks

3. Given this FastAPI code snippet, what will be the output when accessing /users/42?

from fastapi import FastAPI
app = FastAPI()

@app.get("/users/{user_id}")
async def read_user(user_id: int):
    return {"user_id": user_id}

medium

A. {"user_id": 42}

B. TypeError because user_id is not a string

C. 404 Not Found error

D. {"user_id": "42"}

Solution

1. Step 1: Understand parameter type conversion

   The path parameter user_id is declared as int, so FastAPI converts the string "42" from the URL to integer 42.

2. Step 2: Check the returned JSON

   The function returns a dictionary with key "user_id" and value 42 as an integer, which FastAPI converts to JSON.

3. Final Answer:

   {"user_id": 42} -> Option A

4. Quick Check:

   Path param type int converts "42" to 42 [OK]

Hint: Check parameter type hints to know output data type [OK]

Common Mistakes:

• Assuming path parameters are always strings
• Expecting a 404 error without reason
• Confusing JSON string with Python dict

4. What is wrong with this FastAPI path parameter code?

from fastapi import FastAPI
app = FastAPI()

@app.get("/products/{product_id}")
async def get_product(product_id):
    return {"product": product_id}

medium

A. Function must be synchronous, not async

B. Missing type annotation for product_id

C. Incorrect route path syntax

D. Return statement must be a string, not dict

Solution

1. Step 1: Check function parameter declaration

   The parameter product_id lacks a type hint, so FastAPI treats it as a string by default but it's best practice to specify type.

2. Step 2: Identify the issue

   While the code runs, missing type annotation can cause unexpected behavior or lack of validation.

3. Final Answer:

   Missing type annotation for product_id -> Option B

4. Quick Check:

   Always add type hints for path parameters [OK]

Hint: Always add type hints to path parameters for validation [OK]

Common Mistakes:

• Ignoring type hints thinking they are optional
• Confusing async with sync function requirements
• Thinking route path syntax is wrong when it is correct

5. You want to create a FastAPI route that accepts two path parameters: category (string) and item_id (integer). Which of the following is the correct way to define this route and function?

hard

A. @app.get("/items/{category}/{item_id}")
async def read_item(category: str, item_id: int):
return {"category": category, "item_id": item_id}

B. @app.get("/items/{category}/{item_id}")
async def read_item(item_id: int, category: str):
return {"category": category, "item_id": item_id}

C. @app.get("/items/{category}/{item_id}")
async def read_item(category, item_id):
return {"category": category, "item_id": item_id}

D. @app.get("/items/{category}/{item_id}")
def read_item(category: str, item_id: int):
return {"category": category, "item_id": item_id}

Solution

1. Step 1: Match path parameters with function parameters

   The route declares two path parameters in order: category then item_id. The function parameters must match this order and names.

2. Step 2: Check type annotations and async usage

   @app.get("/items/{category}/{item_id}") async def read_item(category: str, item_id: int): return {"category": category, "item_id": item_id} correctly uses type hints (str and int) and async function, which is recommended in FastAPI.

3. Final Answer:

   @app.get("/items/{category}/{item_id}") async def read_item(category: str, item_id: int): return {"category": category, "item_id": item_id} -> Option A

4. Quick Check:

   Order and types must match route parameters [OK]

Hint: Order and names of parameters must match route path [OK]

Common Mistakes:

• Swapping parameter order in function
• Omitting type hints for path parameters
• Using sync function without reason