Path parameters — {id} becomes a function arg

Wrap a segment of the path in {braces} and FastAPI extracts it. Type hints validate.

main.py v3.1

from fastapi import FastAPI, HTTPException

app = FastAPI()

# Path param with int validation
@app.get('/items/{item_id}')
async def get_item(item_id: int):
    return {'item_id': item_id}

# Multiple path params
@app.get('/users/{user_id}/posts/{post_id}')
async def get_post(user_id: int, post_id: int):
    return {'user_id': user_id, 'post_id': post_id}

# Path param with str (no validation — accepts anything)
@app.get('/search/{query}')
async def search(query: str):
    return {'query': query, 'results': []}

# Path param with enum-like choices
from enum import Enum
class Color(str, Enum):
    red = 'red'
    green = 'green'
    blue = 'blue'

@app.get('/colors/{color}')
async def get_color(color: Color):
    return {'color': color, 'value': color.value}

# 404 if item doesn't exist
fake_db = {1: 'apple', 2: 'banana'}
@app.get('/fruits/{fruit_id}')
async def get_fruit(fruit_id: int):
    if fruit_id not in fake_db:
        raise HTTPException(status_code=404, detail='Fruit not found')
    return {'fruit_id': fruit_id, 'name': fake_db[fruit_id]}

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🧠 How it works

{param} in path → param becomes a function arg. Type hints validate: int — /items/abc returns 422. str — anything works. float — /items/1.5 works. bool — /items/true, /items/1, /items/True work. UUID — /items/123e4567-e89b-12d3-a456-426614174000. Enum types constrain choices — invalid values return 422. Order matters: /users/me should be defined BEFORE /users/{user_id} (else 'me' is caught by the param route). HTTPException(status_code, detail) raises an HTTP error with a custom message.

✏️ Try these changes

Change item_id: int to item_id: str
→ /items/abc now works
Add a new route /products/{product_id} that returns a fake product
→ new endpoint
Change the Color enum to add 'yellow'
→ /colors/yellow works
Change HTTPException status_code to 400
→ Bad Request instead of Not Found

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Your first route — @app.get('/path')

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Query params — ?key=value after the path