Path Parameters: Turning URL Parts into Variables
Path parameters turn parts of a URL, like /users/123, into typed function arguments. FastAPI uses this to create endpoints for specific resources, like fetching a user by their ID. The footgun is forgetting type hints; without int, 123 is just a string.
Why it exists
Web applications need to handle requests for specific, individual items. Hardcoding an endpoint for every single item (/items/1, /items/2, etc.) is impossible. Path parameters solve this by creating a template for an endpoint that can handle any unique identifier.
The mental model
Think of a path parameter as a fill-in-the-blank in your URL. You define a route like /users/{user_id}. When a request comes in for /users/42, FastAPI takes the 42, sees it corresponds to the {user_id} blank, and passes it as an argument to your handler function. It connects the URL structure directly to your Python function's signature.
How it works
In FastAPI, you declare a path parameter using curly braces in the decorator string, like @app.get("/items/{item_id}"). Then, you add a parameter with the exact same name to your function definition: def read_item(item_id):. FastAPI automatically links them. By adding a type hint, like item_id: int, FastAPI will also validate that the incoming path segment is a valid integer and convert it from a string before calling your function. If it's not a valid integer (e.g., /items/foo), FastAPI returns a 422 Unprocessable Entity error automatically.
When to use it
Use path parameters to identify a unique resource. This is the cornerstone of RESTful API design. Common examples include fetching a user by ID (/users/{user_id}), retrieving a specific product (/products/{product_id}), or accessing a blog post by its unique slug (/posts/{post_slug}).
When not to use it
Do not use path parameters for optional filtering, sorting, or pagination. Those belong in query parameters (e.g., /items?limit=10&sort=desc). A path parameter identifies the resource itself; query parameters describe how you want to view or filter a collection of resources. Using a path parameter for optional data makes your URL structure rigid and non-standard.
One canonical example
In a FastAPI app, you define a route with @app.get("/users/{user_id}"). Your function signature must match: def read_user(user_id: int):. A request to the URL /users/123 will execute the read_user function, with the integer 123 automatically validated and passed in as the user_id argument. The function could then return a JSON response like {"user_id": 123}.
Interview question
In FastAPI, what is the primary consequence of defining a path parameter without a type hint (e.g., item_id instead of item_id: int)?
- a.FastAPI will automatically infer the correct data type from the incoming URL segment.
- b.FastAPI will return a 422 Unprocessable Entity error for any request to that endpoint.
- c.The path parameter will always be treated as a string, regardless of its content.Correct
- d.The application will fail to start due to a type declaration error.
Why? this is the answer
The card explicitly states that 'without int, 123 is just a string,' meaning FastAPI treats untyped path parameters as strings by default. FastAPI does not infer types for path parameters; it requires explicit type hints for validation and conversion.
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