Path parameters in FastAPI

Introduction

Path parameters let you get values directly from the URL. This helps your app respond differently based on what the user asks for.

You want to get a user ID from the URL to show their profile.

You need to fetch a product by its ID from the URL.

You want to create URLs that change based on the item requested.

You want to build REST APIs where resources are identified by URL parts.

Syntax

FastAPI

from fastapi import FastAPI

app = FastAPI()

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

Use curly braces {} in the path to mark a path parameter.

Define the parameter in the function with the same name and type.

Examples

Get a user ID from the URL and return it.

FastAPI

from fastapi import FastAPI

app = FastAPI()

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

Use :path to capture a path parameter that can include slashes.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/files/{file_path:path}")
async def read_file(file_path: str):
    return {"file_path": file_path}

Path parameter can be string type too.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/orders/{order_id}")
async def get_order(order_id: str):
    return {"order_id": order_id}

Sample Program

This app returns a message with the book ID taken from the URL path.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/books/{book_id}")
async def read_book(book_id: int):
    return {"book_id": book_id, "message": f"You requested book {book_id}"}

Important Notes

Path parameters are required by default. The URL must include them.

FastAPI converts the parameter to the declared type automatically.

Use descriptive names for path parameters to keep URLs clear.

Summary

Path parameters let you get values from the URL path.

Use curly braces {} in the route to declare them.

They help make your API flexible and dynamic.

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