Path parameter types and validation in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a path parameter in FastAPI?

A path parameter is a variable part of the URL path that FastAPI extracts and passes to your function. It lets you capture values directly from the URL.

beginner

How do you specify the type of a path parameter in FastAPI?

You specify the type by adding a type hint in the function parameter, like item_id: int. FastAPI uses this to validate and convert the value automatically.

intermediate

What happens if a path parameter value does not match the declared type in FastAPI?

FastAPI returns a 422 Unprocessable Entity error because the value cannot be converted to the declared type, ensuring only valid data is processed.

intermediate

How can you add validation constraints like minimum or maximum values to path parameters in FastAPI?

You can use Path() from fastapi with arguments like ge (greater or equal) and le (less or equal) to set limits, e.g., item_id: int = Path(..., ge=1, le=100).

beginner

Why is it important to validate path parameters in web applications?

Validation prevents invalid or harmful data from reaching your code, which helps avoid errors, security issues, and unexpected behavior.

In FastAPI, how do you declare a path parameter named 'user_id' as an integer?

A def get_user(user_id: float):

B def get_user(user_id):

C def get_user(user_id: int):

D def get_user(user_id: str):

Using user_id: int tells FastAPI to expect an integer and validate it.

What response does FastAPI give if a path parameter fails type validation?

A 404 Not Found

B 422 Unprocessable Entity

C 200 OK with empty data

D 500 Internal Server Error

FastAPI returns 422 when the input cannot be converted to the declared type.

Which FastAPI function helps add validation constraints like minimum or maximum values to path parameters?

A Path()

B Body()

C Query()

D Depends()

Path() is used to add validation rules to path parameters.

What does Path(..., ge=1, le=10) mean for a path parameter?

A Value must be between 1 and 10 inclusive

B Value must be greater than 10

C Value must be less than 1

D Value can be any number

The parameter value must be greater or equal to 1 and less or equal to 10.

Why should you use type hints and validation for path parameters?

A To avoid writing functions

B To make URLs longer

C To slow down the app

D To improve code readability and ensure correct data

Type hints and validation help catch errors early and make code easier to understand.

Explain how FastAPI uses type hints to validate path parameters and what happens if validation fails.

Think about how FastAPI checks the URL parts against your function's expected types.

Describe how to add minimum and maximum value constraints to a path parameter in FastAPI and why this is useful.

Consider how you can tell FastAPI to accept only numbers within a certain range.