Path Parameter Basics 🛣️

Path parameters are variables in the URL that let you pass dynamic values to your endpoints.

What are Path Parameters?

Path parameters are parts of the URL that change. For example:

/users/123        ← 123 is the path parameter
/posts/456        ← 456 is the path parameter
/items/my-item    ← my-item is the path parameter

Creating Endpoints with Path Parameters

from fastapi import FastAPI

app = FastAPI()

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

Now you can visit:

• `http://localhost:8000/users/123` → Returns `{"user_id": 123}`

• `http://localhost:8000/users/456` → Returns `{"user_id": 456}`

Type Matters

FastAPI validates types automatically:

@app.get("/users/{user_id}")
async def get_user(user_id: int):
    # user_id MUST be an integer
    # http://localhost:8000/users/abc → Error
    return {"user_id": user_id}

Multiple Path Parameters

You can have multiple parameters:

@app.get("/users/{user_id}/posts/{post_id}")
async def get_user_post(user_id: int, post_id: int):
    return {
        "user_id": user_id,
        "post_id": post_id
    }

Visit: `http://localhost:8000/users/123/posts/456`

Returns: `{"user_id": 123, "post_id": 456}`

String Parameters

Path parameters can be strings:

@app.get("/files/{filename}")
async def get_file(filename: str):
    return {"filename": filename}

Visit: `http://localhost:8000/files/document.pdf`

Returns: `{"filename": "document.pdf"}`

Parameter Order Matters

The URL path should match your parameter names:

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

# ❌ Wrong - parameter order in URL matters
@app.get("/posts/{post_id}/users/{user_id}")
# Must have post_id before user_id in URL

Real-World Example

@app.get("/api/v1/users/{user_id}")
async def get_user(user_id: int):
    # Fetch from database
    user = db.get_user(user_id)
    if not user:
        return {"error": "User not found"}
    return user

@app.get("/api/v1/posts/{post_id}/comments/{comment_id}")
async def get_comment(post_id: int, comment_id: int):
    comment = db.get_comment(post_id, comment_id)
    return comment

🔑 Key Takeaways

• ✅ Use `{variable_name}` in routes for path parameters

• ✅ Type hints validate the parameter type

• ✅ Multiple parameters work in the same path

• ✅ Order of parameters matters

• ✅ Strings and integers are most common types

• ✅ Path parameters are always required